#include <iostream>
#include <vector>

using namespace std;

int main(void){
    int N; cin >> N;
    long int max=0, ans=0;
    vector <int> v(N);
    for(int i=0; i<N; i++){
        int a,b;
        cin >> a>> b;
        v[i] = a + b*4;
        if(max< v[i]) max = v[i];
    }
    for(int i=0; i<N; i++){
        if((max-v[i])%2==1){
            cout << -1 << endl;
            return 0;
        }
        ans += (max-v[i])/2;
    }
    cout << ans << endl;
    return 0;
}