#include <bits/stdc++.h>
using namespace std;

int main() {
    int N;
    set<int> S;
    cin >> N;
    while (N--) {
        int x, y;
        cin >> x >> y;
        S.insert(y - x);
    }
    int ans = *S.begin();
    if (ans <= 0 || S.size() != 1)
        ans = -1;
    cout << ans << endl;
}