#include <iostream>
#include <vector>

using namespace std;

int main() {

    std::ios::sync_with_stdio(false);
    std::cin.tie(0);

    int n;
    cin >> n;
    int x = 0;
    int y = 0;
    vector<int> vec;
    int ans = -1;
    for (int i = 0; i < n; ++i) {
        cin >> x >> y;
        if (x >= y) {
            break;
        } else {
            vec.push_back(y - x);
        }
    }
    if (!vec.empty()) {
        ans = vec.at(0);
        for (size_t j = 0; j < vec.size(); ++j) {
            if (ans != vec.at(j)) {
                ans = -1;
                break;
            }
        }
    }
    cout << ans << endl;
    return 0;
}