Fast Number Operations (Dev C++ Code)

/*-----------------------------------------------------------
 * Description: Program that executes fast number operations on three numbers.
 *-----------------------------------------------------------*/
#include <stdio.h>
int displayMenu(void);    // Functions Prototypes
void displayNumbers(void);
void sortNumbers(void);
double avgNumbers(void);
int minNumbers(void);
int maxNumbers(void);
int prime(int);
int perfect(int);
void input(void);
void init(void);
int n1=-1,n2=-1,n3=-1,choice; // n1,n2,n3 3 number type int initialized to -1 ; choice => is the menu choice 
int main(void) 
{
menu:	
	switch (displayMenu()) // switch to make the appropriate choice 
	{
		case 1:
			input();
			goto menu; // goto statement to take us back to menu 
			break;
		case 2:
			(n1==-1) ? puts("N1 : -1 N2 : -1 N3: -1") : displayNumbers();
			goto menu;
			break;
		case 3:
			sortNumbers();
			(n1==-1) ? puts("ERROR: No numbers entered.") : printf("The numbers after the sort are :\n->N1: %d\n->N2: %d\n->N3: %d\n ",n1,n2,n3);
			goto menu;
			break;
		case 4:
			(n1==-1) ? puts("ERROR: No numbers entered.") : printf("The average of the 3 numbers is %.2f\n",avgNumbers());
			goto menu;
			break;
		case 5:
			(n1==-1) ? puts("ERROR: No numbers entered.") : printf("The max is %d and the min is %d\n",maxNumbers(),minNumbers());
			goto menu;
			break;
		case 6:
			(n1==-1) ? puts("ERROR: No numbers entered.") : printf(" -> %d is %s\n -> %d is %s\n -> %d is %s\n",n1, prime(n1)==1 ? "a prime number" :"not a prime number",n2,prime(n2)==1 ? "a prime number" :"not a prime number",n3,prime(n3)==1 ? "a prime number" :"not a prime number");
			goto menu;
			break ; 
		case 7 :
			(n1==-1) ? puts("ERROR: No numbers entered.") : printf(" -> %d is %s\n -> %d is %s\n -> %d is %s\n",n1, perfect(n1)==1 ? "a perfect number" :"not a perfect number",n2,perfect(n2)==1 ? "a perfect number" :"not a perfect number",n3,perfect(n3)==1 ? "a perfect number" :"not a perfect number");
			goto menu;
			break ; 
		case 8 :
			init();
			goto menu;
			break ; 
		case 9:
			return 0;	
		default :
			puts("ERROR: Incorrect command, try again");
			goto menu;	
	}
	return 0;
}
int displayMenu(void)   
{
	puts("******** Main menu ********");
	puts("1 : Input the numbers");
	puts("2 : Display the numbers");
	puts("3 : Sort the numbers");
	puts("4 : Display the average");
	puts("5 : Display the min and max");
	puts("6 : Display Prime or Not");
	puts("7 : Display Perfect or Not");
	puts("8 : Initialize the numbers");
	puts("9 : Quit");
	puts("**************************");
	printf("Please, enter your choice > ");
	scanf("%d",&choice);
	return choice;
}
void input(void) // get 3 numbers from the user and check for validity 
{
	puts("Enter 3 numbers :");
try_n1 :	
	printf("The first number  => ");
	scanf("%d",&n1);
	if (n1 < 0)
	{
		puts("ERROR: Incorrect number, try again");
		goto try_n1;
	} 
try_n2:	
	printf("The second number => ");
	scanf("%d",&n2);
	if (n2<0)
	{
		puts("ERROR: Incorrect number, try again");
		goto try_n2;
	}
try_n3:
	printf("The third number  => ");
	scanf("%d",&n3);
	if (n2<0)
	{
		puts("ERROR: Incorrect number, try again");
		goto try_n3;
	}
}
int minNumbers(void) // funtion to find  min number
{
	int min=n1;  // initialize  min to n1 to make the comparison easier 
	(n2<min) ? min=n2 : min;   // if n2 is less than min we assign n2 to min
	(n3<min) ? min=n3 : min;	// if n3 is less than min we assign n3 to min
	return min; // retrun the min
}
int maxNumbers(void) // function to find the max number
{
	int max=n1;
	(n2>max) ? max=n2 : max;	 
	(n3>max) ? max=n3 : max;
	return max;
}
double avgNumbers(void) // compute the avg
{
	return (n1+n2+n3)/3.0; // return the avg
}
void sortNumbers(void) // sort n1 n2 n3
{
	int x,y,z;
	x=maxNumbers();
	z=minNumbers();
	y=(n1+n2+n3)-(x+z);
	n1=z;n2=y;n3=x;
}
int prime(int n) // prime function
{
	int x=1,i;  
	if (n>1)  // We know that prime numbers are > than 0 and 1 is not a prime number  
	{
		for (i=2;i<=(n-1);i++) // We loop for different divisors ; we put "i=2" because every number is divisible by 1 no need to cover it 
		{
			if (n%i==0)  // if the condition is ture add 1 to X
				{
					x=0;
					break;    // include "break" is we find divisors 
				}
		}	
	}
	else 
		x=0;  // case of 0 or 1 (not prime numbers)
	return x;
}
int perfect(int n) // perfect function
{
	int i,sum=0;   
	for(i=1; i<n ; i++)     // loop for divisors 
      (n%i==0) ? sum=sum+i : sum;  //conditional operator to compute the sum 
    if (n>1)  
  		(sum==n) ? n=n-(n-1) : n;  // if sum=number it's a perfect number
  	else 
	  	n=0;	
	return n;
}
void init(void) // function to initialize numbers to -1
{
	n1=n2=n3=-1;
}
void displayNumbers(void)
{
	printf("->N1: %d\n->N2: %d\n->N3: %d\n",n1,n2,n3);
}
/*-------------------------------------End----------------------*/