#include <stdio.h>

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