#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
long double combine(int x,int y,int n,int m){
	long double sum=1;
	if(2*y>x)
		y=x-y;
	if(2*m>n)
		m=n-m;
	for(int i=y,j=x,k=m,l=n;i>=1||k>=1;--i,--j,--k,--l){
		if(i<1)i=1,j=1;
		if(k<1)k=1,l=1;
		sum*=(long double)(j*k)/(i*l);
	}
	return sum;
}

int main(){
	int p,q,r,s;
	long double sun;
	freopen("in.txt","r",stdin);
	freopen("ou.txt","w",stdout);
	while(cin>>p>>q>>r>>s){
		sun=combine(p,q,r,s);
		printf("%.5lf\n",sun);
	}
	return 0;
}