#include<iostream>
#include<string>
#include<algorithm>
#include<cctype>
#include<set>
#include<bitset>
#include<math.h>
#include<map>
#include<queue>
#include<iomanip>
using namespace std;

int main(){
    int n, max1 = -1, sum1 = 0, oe = -1;
    cin >> n;
    for (int i = 0; i < n; i++){
        int a, b, tmp;
        cin >> a >> b;
        if (i == 0) oe = a%2;
        tmp = a + b*4;
        if(a%2 != oe){
            cout << -1 << endl;
            return 0;
        } else {
            max1 = max(max1, tmp);
            sum1 += tmp;
        }
    }
    cout << (max1*n - sum1)/2 << endl;
}