#include <cstdio>

const int NX = 101;

int N;
int A[ NX ];

int main()
{
  scanf( "%d", &N );
  for( int i = 0; i < N; ++i ) scanf( "%d", A+i );

  int sol = 0;

  for( int i = N-2; i >= 0; --i )
    if( A[i] >= A[i+1] )
    {
      sol += A[i] - A[i+1] + 1;
      A[i] = A[i+1] - 1;
    }

  printf( "%d\n", sol );
}