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

int s[1010];
int t[1010];

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

    int tmax = 0;
    int ans = -1;

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

        int x = a + b * 4;
        tmax = std::max(tmax, x);
    }

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

    if(sum % 2 == 0){
        ans = sum / 2;
    }

    cout << ans << endl;
}