#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);

    int N; cin >> N;
    vector<int> c(N);
    int ma = 0;
    rep(i,N){
        int a,b; cin >> a >> b;
        c[i] = a + 4 * b;
        ma = max(ma, c[i]);
    }

    ll ans = 0;
    rep(i,N){
        if((ma - c[i]) % 2 == 1){
            cout << -1 << endl;
            return 0;
        }
        ans += (ma - c[i]) / 2;
    }

    cout << ans << endl;
}