#include <stdio.h>
#include <string.h>

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