#include <iostream>
using namespace std;

int main() {
	int n, x, y, c = -1;
	
	cin >> n;
	for(int i=0;i<n;++i){
		cin >> x >> y;
		
		if(c == -1){
			c = y - x;
		}else{
			if(y - x <= 0 || y - x != c){
				cout << -1 << endl;
				return 0;
			}
		}
	}
	cout << c << endl;
	
	return 0;
}