#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define f(i,n) for(ll i=0;i<n;i++)
#define m(a,b,c) max(a,max(b,c))
#define N ll n;cin >> n;
#define Nk ll n,k;cin >> n >> k;
#define A ll a[n];f(i,n) cin >> a[i];
#define FAST_IO ios_base::sync_with_stdio(false), cin.tie(nullptr);
#define YES cout<<"YES"<<endl;
#define NO cout<<"NO"<<endl;
#define S string s;cin>>s;
#define pb push_back

int main()
{
   FAST_IO;
   N;
   A;
   vector<ll>v;
   f(i,n)
   {
       if(a[i]%2)
       {
           v.pb(i);
       }
   }
   if(v.size()%2)
   {
       cout<<-1;
   }
   else
   {
       ll s=0;
       for(ll i=0;i<v.size();i=i+2)
       {
           s+=v[i+1]-v[i]+1;
       }
       cout<<n-s+v.size()/2;
   }
   
   return 0;
}