One pass is indeed enough!

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;

/**
 * 
 * @author Sanchit M. Bhatnagar
 * @see http://uhunt.felix-halim.net/id/74004
 * 
 */
public class P11683 {

  public static void main(String[] args) throws Exception {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    PrintWriter out = new PrintWriter(System.out);
    StringTokenizer st = null;
    String line;

    while ((line = br.readLine()) != null) {
      st = new StringTokenizer(line.trim());
      int A = Integer.parseInt(st.nextToken());
      if (A == 0)
        break;

      int B = Integer.parseInt(st.nextToken());
      int[] sculpt = new int[B];
      st = new StringTokenizer(br.readLine().trim());
      int ans = 0;
      int last = -1;
      for (int i = 0; i < B; i++) {
        sculpt[i] = Integer.parseInt(st.nextToken());
        if (last == -1) {
          last = sculpt[i];
          if (sculpt[i] != A)
            ans += (A - sculpt[i]);
        }
        if (sculpt[i] >= last) {
          last = sculpt[i];
        } else {
          ans += (last - sculpt[i]);
          last = sculpt[i];
        }
      }
      out.println(ans);
    }

    out.close();
    br.close();
  }
}