content: /**
   4 5 6 7
   4 6 7 7
   4 6 8 8
   */
  import java.io.*;
  import java.math.*;
  import java.security.*;
  import java.text.*;
  import java.util.*;
  import java.util.concurrent.*;
  import java.util.regex.*;
  
  public class Solution {
  
    // Complete the fairRations function below.
    static int fairRations(int[] B) {
      int sum = 0;
      for (int i = 0; i < B.length; i++) {
        sum += B[i];
      }
      if (sum % 2 != 0) {
        return -1;
      }
      int counter = 0;
      for (int i = 0; i < B.length - 1; i++) {
        if (B[i] % 2 != 0) {
          B[i]++;
          B[i+1]++;
          counter += 2;
        }
      }
      return counter;
    }
  
    private static final Scanner scanner = new Scanner(System.in);
  
    public static void main(String[] args) throws IOException {
      BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH")));
  
      int N = scanner.nextInt();
      scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");
  
      int[] B = new int[N];
  
      String[] BItems = scanner.nextLine().split(" ");
      scanner.skip("(\r\n|[\n\r\u2028\u2029\u0085])?");
  
      for (int i = 0; i < N; i++) {
        int BItem = Integer.parseInt(BItems[i]);
        B[i] = BItem;
      }
  
      int result = fairRations(B);
  
      if (result == -1) {
        bufferedWriter.write("NO");
      } else {
        bufferedWriter.write(String.valueOf(result));
      }
  
      bufferedWriter.newLine();
  
      bufferedWriter.close();
  
      scanner.close();
    }
  }
  