#include<stdio.h>

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