#include <stdio.h>

int main(void){
	int i,n;
	int x,y;
	int ans = 0;
	
	scanf("%d", &n);
	for(i=0;i<n;i++){
		scanf("%d %d", &x, &y);
		if(ans == 0){
			ans = y-x;
		}else{
			if(ans > 0 && ans == (y-x)){
				
			}else{
				ans = -1;
			}
		}
		
	}
	if(ans <= 0){ans=-1;}
	printf("%d\n", ans);
	return 0;
}