#include<bits/stdc++.h>
#define ll long long 
using namespace std;

int main(){
  ll n, a, ans = 0, sum = 0;
  cin >> n;
  //vector<ll> a(n);
  for (int i = 0; i < n; i++){
    cin >> a;
    sum += a;
    if (sum % 2 == 0){
      ans++;
      sum = 0;
    }
  }
  if (sum) cout << -1 << endl;
  else cout << ans << endl;
}