#include <iostream>
using namespace std;

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