#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <stack>
#include <algorithm>
#include <string>
#include <map>
#include <iterator>
#include <set>
#include <queue>

using namespace std;


int main() {

    int N;
    cin >> N;
    vector<int> vs;
    for (int i = 0; i < N; ++i) {
        int X, Y;
        cin >> X >> Y;

        vs.push_back(Y - X);

    }

    if (vs[0] <= 0) {
        cout << "-1" << endl;
        return 0;
    }
    bool ok = true;
    for (int i = 1; i < N; ++i) {
        if (vs[i] > 0 and vs[i] == vs[i - 1]) {

        } else {
            ok = false;
            break;
        }
    }

    if (ok) {
        cout << vs[0] << endl;
    } else {
        cout << -1 << endl;
    }

    return 0;
}