package chelper;

import java.util.Objects;
import java.util.SortedMap;
import java.util.TreeMap;
import java.util.TreeSet;

import io.InputReader;
import io.OutputWriter;
import misc.SimpleSavingChelperSolution;


public class TaskC extends SimpleSavingChelperSolution {

  public void solve(int testNumber, InputReader in, OutputWriter out) {
    wrapSolve(testNumber, in, out);
  }

  class Pos implements Comparable<Pos> {
    long height;
    int pos;

    public Pos(long height, int pos) {
      this.height = height;
      this.pos = pos;
    }

    @Override
    public int compareTo(Pos o) {
      int t = Long.compare(height, o.height);
      if (t != 0) {
        return t;
      }
      return -Integer.compare(pos, o.pos);
    }

    @Override
    public boolean equals(Object o) {
      if (this == o) return true;
      if (o == null || getClass() != o.getClass()) return false;
      Pos pos1 = (Pos) o;
      return height == pos1.height &&
          pos == pos1.pos;
    }

    @Override
    public int hashCode() {

      return Objects.hash(height, pos);
    }
  }

  @Override
  public void solve(int testNumber) {
    int n = in.nextInt();

    long[] above = in.nextLongArray(n);

    long[] total = new long[n];

    TreeSet<Pos> poses = new TreeSet<>();
    for (int i = 0; i < n; i++) {
      poses.add(new Pos(above[i], i));
    }
//    poses.add(new Pos(-1, -1));

    Pos highest = poses.pollLast();
    total[highest.pos] = highest.height;

//    while (!poses.isEmpty()) {
//      Pos next = poses.pollLast();
//      if (next.pos > highest.pos) {
//        continue;
//      }
//
//      long h = highest.height;
//
//      for (int i = highest.pos; i >= Math.max(next.pos, 0); i--) {
//        total[i] = h;
//        if (h > next.height) {
//          h--;
//        }
//      }
//
//      highest = next;
//    }

    long h = highest.height;
    for (int pos = highest.pos; pos < n; pos++) {
      poses.remove(new Pos(above[pos], pos));
      total[pos] = h;
    }

    for (int pos = highest.pos - 1; pos >= 0; pos--) {
      if (h > poses.last().height) {
        h--;
      }
      total[pos] = h;
      poses.remove(new Pos(above[pos], pos));
    }

    h = 0;
    long sum = 0;

    for (int i = 0; i < n; i++) {
//      h = Math.max(h, total[i]);
      sum += total[i] - above[i];
    }

    out.println(sum);
  }
}