#include<bits/stdc++.h>
#define rep(i,a,n) for (int i = a;i < n;i++)
using namespace std;

int main(){
    int n; cin >> n;
    int lmax = 0;
    int lmin = 99999999;
    long long lsum = 0;
    rep(i,0,n){
        int a,b;
        cin >> a >> b;
        int len = a + b * 4;
        if(i != 0){
            if(lmax % 2 != len % 2){
                cout << -1 << endl;
                return 0;
            }
        }
        lmax = max(len, lmax);
        lmin = min(len, lmin);
        lsum += len;
    }
    cout << ((long long)lmax * n -lsum ) / 2 << endl;
}