#include <stdio.h>
#include <string.h>

#define MAX_N 10

int dig, freq;

int D[MAX_N][MAX_N];

int memo( int N, int f ) {
  if ( N == 0 ) {
    return ( f >= freq );
  }

  if ( D[N][f] != -1 ) {
    return D[N][f];
  }

  int i, q;

  q = 0;
  for ( i = 0; i < 10; i++ ) {
    q += memo( N - 1, f + ( dig == i ) );
  }
  D[N][f] = q;

  return q;
}

int DP( int X ) {
  char buf[MAX_N];
  int N, i, j, currDig, q, fr;

  sprintf( buf, "%d", X );
  N = strlen( buf );

  q = fr = 0;

  for ( i = 0; i < N; i++ ) {
    currDig = buf[i] - '0';
    for ( j = 0; j < currDig; j++ )
      q += memo( N - i - 1, fr + ( dig == j ) );
    fr += ( currDig == dig );
  }
  return q;
}

int main(void) {
  FILE *fin, *fout;
  int A, B, i, j;

  fin = fopen( "cifre.in", "r" );
  fscanf( fin, "%d%d%d%d", &A, &B, &dig, &freq );
  fclose( fin );

  for ( i = 0; i < MAX_N; i++ )
    for ( j = 0; j < MAX_N; j++ )
      D[i][j] = -1;

  fout = fopen( "cifre.out", "w" );
  fprintf( fout, "%.4f\n", 1.00 * ( DP(B+1) - DP(A) ) / (B-A+1) );
  fclose( fout );
  return 0;
}