# include <stdio.h>

# define  _fin  "cifre.in"
# define  _fout "cifre.out"


int a, b, k, c, sol;


void readf()
{
	freopen(_fin, "r", stdin);
	scanf("%d %d %d %d", &a, &b, &c, &k);
}

void solve()
{
	int i, j, cnt;
	
	for (i=a; i<=b; i++)
	{
		for (j=i, cnt=0; j; cnt += ( j%10 == c ), j/=10);
		sol += ( cnt>=k );
	}
}

void writef()
{
	freopen(_fout, "w", stdout);
	printf("%.4f\n", (double)sol / (double)(b-a+1));
}

int main()
{
	readf();
	solve();
	writef();
	
	return 0;
}