#include <iostream>
#include <algorithm>
using namespace std;

int main(){
	int n;
	cin >> n;
	int x, y;
	cin >> 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(y-x<0 || y-x!=ans){ cout << -1 << endl; return 0;}
	}
	cout << ans << endl;

	return 0;
}