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

int main() {
   
      int n,count=0,sum=0;
      cin>>n;
      int arr[n];
      
      for(int i=0;i<n;i++){
          cin>>arr[i];
          sum=sum+arr[i];
          
          
          if(sum%2==0)
          {
              count++;
              sum=0;
          }
      }
      
      if(sum%2!=0){
          cout<<"-1"<<endl;
      }
      else
      {

         cout<<count<<endl;
      }
     
       
   

    return 0;
}