//MY SOLUTION

import java.io.; import java.util.; import java.text.; import java.math.; import java.util.regex.*;

public class Solution {

public static void main(String[] args) {
    Scanner in = new Scanner(System.in);
    int N = in.nextInt();
    int B[] = new int[N];
    int count=0;
    boolean flag=true;
    for(int B_i=0; B_i < N; B_i++)
    {
        B[B_i] = in.nextInt();
    }
    for(int i=0;i<N;i++)
    {
        if(B[i]%2!=0)
        {
            B[i]++;
            count++;

            if(i==0)
            {
                B[i+1]++;
                count++;
            }                                     
            else if(i==(N-1))
            {
                 B[i-1]++;
                 count++;
            }
            else
            {
                B[i+1]++;
                count++;
            }     
        }    
    }    
    for(int i=0;i<N;i++)
    {
        if(B[i]%2!=0)
        {
            flag=false;
        }    
    }
    if(flag)
        System.out.println(count);
    else
        System.out.println("NO");
}

}