#include<iostream>
#include<algorithm>
#define rep(i,s,n) for(int i=s;i<n;i++)
#define Z 20000
using namespace std;
int d[Z+1];
int main(){
	int n,m,k,r,l;
	while(true){
		cin>>n>>k>>m>>r;
		if(!(n+m+k+r))break;
		rep(i,0,Z)d[i]=0;
		k=1,l=n;
		rep(i,0,Z)d[i]+=k/l,k%=l,k*=10;
		if(m)rep(i,1,n){k=1,l=i*n;rep(j,0,Z)d[j]+=k/l,k%=l,k*=10;}
		for(int i=Z;i>0;i--)d[i-1]+=d[i]/10,d[i]%=10;
		cout<<d[0]<<".";
		rep(i,1,r+1)cout<<d[i];
		cout<<endl;
	}
	return 0;
}