#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <math.h>
using namespace std;

int main() {
    int n;
    cin >> n;
    vector<int> l(n,0);
    for(int i = 0;i < n;i++){
        int a,b;
        cin >> a >> b;
        l[i] = a + 4*b;
    }
    sort(l.begin(),l.end(),greater<int>());
    int ans = 0;
    for(int i = 0;i < n;i++){
        ans += l[0] - l[i];
    }
    if(ans % 2 == 0)cout << ans / 2 << endl;
    else cout << -1 << endl;
    return 0;
}