#include <bits/stdc++.h>
int main(void) {
  int N, sum = 0;
  scanf("%d", &N);
  int A[N];
  for (int i = 0; i < N; i++) {
    scanf("%d", &A[i]);
  }
  for (int i = 1; i < N; i++) {
    int j = 0;
    while (A[i] < A[i - 1]) {
      sum++;
      A[i]++;
    }
  }
  printf("%d\n", sum);
  return 0;
}
