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

int N, sum = 0, mx = 0;
vector<int> a, b, c;

void input() {
  cin >> N;
  a.resize(N);
  b.resize(N);
  c.resize(N);
  rep(i, 0, N) {
    cin >> a[i] >> b[i];
    c[i] = a[i] + b[i] * 4;
    sum += c[i];
    mx = max(mx, c[i]);
  }
}

void solve() {
  int ans = 0;
  if (sum & 1)
    ans = -1;
  else
    rep(i, 0, N) ans += (mx - c[i]) / 2;
  cout << ans << endl;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  input();
  solve();
  getchar();
}