#include <stdio.h>

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