#include <stdio.h>

int main(void)
{
	int i, N, x[1001], y[1001], a;
	scanf("%d", &N);
	for(i = 1;i <= N;i++){
		scanf("%d%d", &x[i], &y[i]);
	}
	a = y[1] - x[1];
	if(a <= 0){
		printf("-1\n");
		return 0;
	}
	for(i = 2;i <= N;i++){
		if(y[i] - x[i] != a){
			printf("-1\n");
			return 0;
		}
	}
	printf("%d\n", a);
	
	return 0;
}