xcode: Move sprite to specific position

[sk8ergirl]

this is my background http://s13.postimg.org/6zejiqc87/1429994667798.png

in current code user can move sprite to any location, but I want the user to only move sprite to specific location. I have one Sprite (s1) I want to move it to specific position as you can see my background have line and dots and these dots are the position I want the user to be able to move the sprite from 1 to 2 and then from 2 to 3 but they can't skip one position for example they can't move sprite from 1 to 3 or from 2 to 4
I don't know how to get the x,y location of the dots in the background

- (id)initWithSize:(CGSize)size {
    if (self = [super initWithSize:size]) {

        // 1) Loading the background
        _background = [SKSpriteNode spriteNodeWithImageNamed:@"mybackground"];
        [_background setName:@"background"];
        [_background setAnchorPoint:CGPointZero];
        [self addChild:_background];

        // 2) Loading the images
     //   NSArray *imageNames = @[@"s1", @"s2"];
          NSArray *imageNames = @[@"s1"];
        for(int i = 0; i < [imageNames count]; ++i) {
            NSString *imageName = [imageNames objectAtIndex:i];
            SKSpriteNode *sprite = [SKSpriteNode spriteNodeWithImageNamed:imageName];
            [sprite setName:kAnimalNodeName];

            float offsetFraction = ((float)(i + 1)) / ([imageNames count] + 1);
            [sprite setPosition:CGPointMake(size.width * offsetFraction, size.height / 2)];
            [_background addChild:sprite];
        }
    }

    return self;
}

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    CGPoint positionInScene = [touch locationInNode:self];

    [self selectNodeForTouch:positionInScene];
}