#include <bits/stdc++.h>

#pragma GCC optimize("Ofast")
using namespace std;

void solve(int cs) {
  int n;
  cin >> n;
  int res = 0;
  int cnt = 0;
  int odd = 0;
  int odds = 0;
  for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    if (x % 2) {
      odds += 1;
      if (odd) {
        odd = 0;
        res += 1;
      } else {
        odd = 1;
      }
    } else {
      if (!odd) res += 1;
    }
  }
  cout << ((odds % 2) ? -1 : res) << "\n";
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int tc = 1;
  // cin >> tc;
  for (int cs = 1; cs <= tc; cs++) {
    solve(cs);
  }
  return 0;
}