#include <iostream>
#include <algorithm>
using namespace std;
 
int main(){
	int N, X, Y;
	cin >> N >> X >> Y;
	int ans = Y - X;
	if(ans <= 0){
		cout << -1 << endl;
		return 0;
	}
	for(int i = 1; i < N; i++){
		cin >> X >> Y;
		if(ans + X != Y){
			cout << -1 << endl;
			return 0;
		}
	}
	cout << ans << endl;
}