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

int main()
{
    int n;
    cin >> n;
    int a,b,cnt = 0,size[n] = {};
    for(int i = 0; i < n; i++){
        cin >> a >> b;
        size[i] += a + b * 4;
        if(a % 2 == 1) cnt++;
    }

    int max = *max_element(size,size+n);
    long long ans = -1;
    if(cnt == n && cnt % 2 == 0){
        ans++;
        for(int i = 0; i < n; i++){
            ans += (max - size[i]) / 2;
        }
    }

    cout << ans << endl;

    return 0;
}