#include <iostream>

using namespace std;

int main() {
    int n, x, y;
    int ans;
    
    cin >> n;
    
    for (int i = 0; i < n; i++) {
        cin >> x >> y;
        if (y - x <= 0 || (i != 0 && ans != y - x)) {
            ans = -1;
            break;
        }
        ans = y - x;
    }
    
    cout << ans << endl;
    
    return 0;
}