#include <bits/stdc++.h>

#define rep(i, n) for (int (i) = 0; (i) < (int)(n); (i)++)

const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, 1, 0, -1};
using namespace std;
typedef long long ll;

const int INF = 1e9;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int N;
    cin >> N;
    int ans = -INF;
    for (int i = 0; i < N; i++) {
        int x, y;
        cin >> x >> y;
        if (ans == -INF) ans = y-x;
        if (ans >= 1) {
            if (ans != y-x) ans = -1;
        }
    }
    if (ans < 0) ans = -1;
    cout << ans << endl;
    return 0;
}