#include <bits/stdc++.h>

using namespace std;

typedef unsigned long long ull;
typedef long long ll;

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