#include <bits/stdc++.h>
using namespace std;
 
typedef long long ll;
typedef pair<int,int> P;
 
const int INF = 1e9;
const int mod = 1e9+7;
const double EPS = 1e-10;
const double PI = acos(-1.0);

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