#include <iostream>
using namespace std;

int main(){
    int N;
    cin >> N;
    int space[N][2];
    for(int i = 0; i < N; i++){
        cin >> space[i][0];
        cin >> space[i][1];
    }
    int space_odd = space[0][0] % 2;
    for(int i = 1; i < N; i++){
        if(space[i][0] % 2 != space_odd){
            cout << -1 << endl;
            return 0;
        }
    }
    int max_space = 0;
    for(int i = 0; i < N; i++){
        max_space = max(max_space, space[i][0] + space[i][1] * 4);
    }
    int ans = 0;
    for(int i = 0; i < N; i++){
        ans += max_space - space[i][0] - space[i][1] * 4;
    }
    cout << ans / 2 << endl;
}