#include <bits/stdc++.h>

#define rep(i, n) for (int (i) = 0; (i) < (int)(n); (i)++)

const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, 1, 0, -1};
using namespace std;
typedef long long ll;

const int MAXN = 1020;
int len[MAXN];

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