#include <bits/stdc++.h>

using namespace std;

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