#include <iostream>
#include <algorithm>
using namespace std;

long long s[1010];
long long t[1010];

int main(){
    long long n;
    cin >> n;

    long long tmax = 0;
    long long ans = -1;

    for(long long i = 0; i < n; ++i){
        long long a, b;
        cin >> a >> b;
        s[i] = a;
        t[i] = b;

        long long x = a + b * 4;

        if(i != 0){
            if(tmax % 2 != x % 2){
                cout << -1 << endl;
            }
        }

        tmax = std::max(tmax, x);
    }

    long long sum = 0;
    for(long long i = 0; i < n; ++i){
        long long x = tmax - (s[i] + t[i] * 4);
        sum += x;
    }

    cout << sum / 2 << endl;
}