#include <bits/stdc++.h>

#define show(x) cerr << #x << " = " << x << endl

using namespace std;
using ll = long long;

template <typename T>
ostream& operator<<(ostream& os, const vector<T>& v)
{
    os << "sz:" << v.size() << "\n[";
    for (const auto& p : v) {
        os << p << ",";
    }
    os << "]\n";
    return os;
}

template <typename S, typename T>
ostream& operator<<(ostream& os, const pair<S, T>& p)
{
    os << "(" << p.first << "," << p.second
       << ")";
    return os;
}


constexpr ll MOD = (ll)1e9 + 7LL;

template <typename T>
constexpr T INF = numeric_limits<T>::max() / 64;

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n;
    cin >> n;
    vector<int> x(n);
    vector<int> y(n);
    for (int i = 0; i < n; i++) {
        cin >> x[i] >> y[i];
    }
    const int a = y[0] - x[0];
    if (a <= 0) {
        cout << -1 << endl;
        return 0;
    } else {
        for (int i = 1; i < n; i++) {
            if (a + x[i] != y[i]) {
                cout << -1 << endl;
                return 0;
            }
        }
    }
    cout << a << endl;
    return 0;
}