#include <stdio.h>

int main(void) {
	// your code goes here
	int n, i;
	long x, y, ans;
	fscanf(stdin, "%d", &n);
	fscanf(stdin, "%ld %ld", &x, &y);
	ans = y - x;
	if (ans < 1)
		ans = -1;
	for(i = 0; i < n; i++){
		fscanf(stdin, "%ld %ld", &x, &y);
		if(ans != (y - x)){
			ans = -1;
			break;
		}
	}
	printf("%ld\n", ans);
	return 0;
}