#include <cstdio>

int A[104], N;
int result;

int main(void) {
  scanf("%d", &N);
  for ( int i = 0; i < N; ++i )
    scanf("%d", A + i);
  
  
  bool change;
  do {
    change = false;
    for ( int i = 0; i < N-1; ++i ) {
      if ( A[i] >= A[i+1] ) {
        change = true;
        result += A[i] - A[i+1] + 1;
        A[i] = A[i+1] - 1;
      }
    }
  } while ( change );
  
  printf("%d\n", result);
  return 0;
}