#include <bits/stdc++.h>
using namespace std;
int main(){
    int n; cin >> n;
    int x[1010],y[1010],ans[1010]={0};
    int sum=0;
    for(int i=0; i < n; i++) cin >> x[i] >> y[i];
    for(int i=0; i < n; i++){
        if(y[i] - x[i] > 0){
            ans[i] = y[i] - x[i];
            sum += ans[i];
        }
    }
    if(sum % n == 0) cout << ans[0] << endl;
    else cout << -1 << endl;
}