#include <stdio.h>

int main(){
	int val = 0;
	scanf("%d",&val);
	
	int print = 0;
	for(int i = 0;i < val;i++){
		int right,ans;
		scanf("%d",&right,&ans);
		if(ans - right < 0){
			printf("-1");
			return 0;
		}else{
			print = ans - right;
			if(print != ans - right){
				printf("-1");
				return 0;
			}
		}
	}
	printf("%d",print);
}