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

int N;

int solve() {
    cin >> N;
    set<int> se;
    for (int i = 0; i < N; ++i) {
        int x, y;
        cin >> x >> y;
        se.insert(y-x);
    }
    if (se.size() > 1) return -1;
    int res = *se.begin();
    if (res > 0) return res;
    else return -1;
}


int main() {
    cout << solve() << endl;
}