백준 2847

package baekjoon.그리디;

import java.io.BufferedReader;
import java.io.InputStreamReader;

class Main2847 {

   public static void main(String[] args) throws Exception {
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
      int N = Integer.parseInt(br.readLine());
      int[] score = new int[N];
      for (int i = 0; i < N; i++) {
         score[i] = Integer.parseInt(br.readLine());
      }
      if (score.length == 1) {
         System.out.println(0);
         return;
      }
      int ans = 0;
      for (int i = N - 1; i > 0; i--) {
         if (score[i - 1] >= score[i]) {
            ans += score[i - 1] - (score[i] - 1);
            score[i - 1] = score[i] - 1;
         }
      }

      System.out.println(ans);
   }
}