source_code: import java.util.*;
  import java.io.*;
  public class Waw{
      
      public static void main(String[] args) throws Exception {
          Scanner sc = new Scanner(System.in);
          int n = sc.nextInt();
          long[] a = new long[n];
          for(int i=0;i<n;i++) a[i] = sc.nextLong();
          long[] p = new long[n];
          p[n-1] = a[n-1];
          for(int i=n-2;i>=0;i--){
              if(a[i]<p[i+1]) p[i] = p[i+1]-1;
              else p[i] = a[i];
          }
          long max = p[0];
          long res = p[0] - a[0];
          for(int i=1;i<n;i++){
              if(max < p[i]) max = p[i];
              res += max - a[i];
          }
          System.out.println(res);
      }
  }