#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

int main() {
  int N;
  scanf("%d", &N);

  vector<ll> s(N);
  ll ma = 0;
  for (int i = 0; i < N; ++i) {
    ll a, b;
    scanf("%lld %lld", &a, &b);
    ll t = a + (b * 4);
    ma = max(ma, t);
    s[i] = t;
  }

  ll ans = 0;
  for (int i = 0; i < N; ++i) {
    if ((ma - s[i]) % 2 != 0) {
      puts("-1");
      return 0;
    }
    ans += (ma - s[i]) / 2;
  }
  printf("%lld\n", ans);
  return 0;
}