#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <set>
#include <map>
#include <iomanip>
using namespace std;
using ll = long long;

int main() {
    int N;
    cin >> N;
    vector<ll> v(N);
    ll ma = 0;
    for(int i=0; i<N; ++i) {
        ll a, b;
        cin >> a >> b;
        v[i] = a + 4*b;
        ma = max(ma, v[i]);
    }
    ll res = 0;
    for(int i=0; i<N; ++i) {
        if((ma - v[i]) % 2 != 0) {
            res = -1;
            break;
        } else {
            res += (ma-v[i])/2;
        }
    }
    cout << res << endl;
}