#include <iostream>

#define REP(i, a, b) for (int i = (a); i < (b); i++)
using namespace std;

int main () {
    int N;
    cin >> N;
    int X, Y, ans;
    cin >> X >> Y;
    ans = Y - X;
    bool ok = ans > 0;
    REP (i, 1, N) {
	cin >> X >> Y;
	if (ans != Y - X) ok = false;
    }
    cout << (ok ? ans : -1) << endl;
    return 0;
}