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);
}
}
알고리즘 & 자료구조/백준