#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <algorithm>
#include <functional>
#include <limits>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <complex>

using namespace std;

#define INF (INT_MAX/3)
#define PI (2*acos(0.0))
#define EPS (1e-8)

typedef long long ll;
typedef unsigned long long ull;

int main(){
    ios_base::sync_with_stdio(0);
    int N, x, y;
    cin >> N >> x >> y;
    int ans = y - x;
    for(int i = 1; i < N; i++){
        cin >> x >> y;
        if(y - x != ans){ ans = -1; break; }
    }
    if(ans < 0) cout << -1 << endl;
    else cout << ans << endl;
    return 0;
}