// No.185 和風
// https://yukicoder.me/problems/no/185
//
#include <iostream>
#include <vector>
using namespace std;
int solve(vector<pair<int, int>> &equations);


int main() {
    unsigned int N;
    cin >> N;

    vector<pair<int, int>> equations;
    for (auto i = 0; i < N; i++) {
        int x, y;
        cin >> x >> y;
        equations.push_back(make_pair(x, y));
    }

    int ans = solve(equations);
    cout << ans << endl;
}

int solve(vector<pair<int, int>> &equations) {
    int ans = equations[0].second - equations[0].first;
    if (ans <= 0)
        return -1;

    for (auto i = 1; i < equations.size(); i++) {
        int tmp = equations[i].second - equations[i].first;
        if (tmp != ans) {
            ans = -1;
            break;
        }
    }
    return ans;
}