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

int main() {
    int n;
    cin >> n;
    int dif;
    int x,y;
    cin >> x >> y;
    dif = y - x;
    dif = dif < 0 ? -1 : dif;
    for(int i = 1;i < n;i++){
        cin >> x >> y;
        if(dif != y - x){
            dif = -1;
            break;
        }
    }
    cout << dif << endl;
    return 0;
}