#include <bits/stdc++.h>
#define FOR(i,bg,ed) for(ll i=(bg);i<(ed);i++)
#define REP(i,n) FOR(i,0,n)
#define MOD 1000000007
#define int long long
using namespace std;
typedef long long ll;
typedef vector<vector<ll>> mat;
const int INF = 1e9;



signed main()
{
    int N;
    int X, Y;

    cin >> N;
    cin >> X >> Y;
    int ans = Y - X;
    if (ans <= 0) {
        cout << -1 << endl;
        return 0;
    }

    REP(i,N-1) {
        cin >> X >> Y;
        if (ans != Y - X) {
            cout << -1 << endl;
            return 0;
        }
        ans = Y - X;
        if (ans <= 0) {
            cout << -1 << endl;
            return 0;
        }
    }

    cout << ans << endl;
}