#include <iostream>
#include <sstream>
#include <iomanip>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <algorithm>
#include <numeric>
#include <iterator>
#include <functional>
#include <complex>
#include <queue>
#include <stack>
#include <cmath>
#include <cassert>
#include <tuple>
using namespace std;
typedef long long LL;
typedef complex<double> CMP;

class FixedDiceGameDiv1 { public:
	double getExpectation(int a, int b, int c, int d)
	{
		if(a*b<=c*1)
			return -1.0;
		auto d1 = distr(a, b);
		auto d2 = distr(c, d);

		double total_prob = 0.0;
		for(auto& vp: d1)
		for(auto& zq: d2)
		{
			int v = vp.first;
			double p = vp.second;
			int z = zq.first;
			double q = zq.second;
			if(v > z) 
				total_prob += p*q;
		}

		double e = 0.0;
		for(auto& vp: d1)
		for(auto& zq: d2)
		{
			int v = vp.first;
			double p = vp.second;
			int z = zq.first;
			double q = zq.second;
			if(v > z) 
				e += v * p*q/total_prob;
		}
		return e;
	}

	map<int, double> distr(int n, int f)
	{
		if(n == 0) {
			map<int, double> m;
			m[0] = 1,0;
			return m;
		}
		map<int, double> m = distr(n-1, f), m2;
		for(auto& up: m) {
			int u = up.first;
			double p = up.second;
			for(int v=1; v<=f; ++v)
				m2[u+v] += p/f;
		}
		return m2;
	}
};

// BEGIN CUT HERE
#include <ctime>
double start_time; string timer()
 { ostringstream os; os << " (" << int((clock()-start_time)/CLOCKS_PER_SEC*1000) << " msec)"; return os.str(); }
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v)
 { os << "{ ";
   for(typename vector<T>::const_iterator it=v.begin(); it!=v.end(); ++it)
   os << '\"' << *it << '\"' << (it+1==v.end() ? "" : ", "); os << " }"; return os; }
void verify_case(const double& Expected, const double& Received) {
 bool ok = (abs(Expected - Received) < 1e-9);
 if(ok) cerr << "PASSED" << timer() << endl;  else { cerr << "FAILED" << timer() << endl;
 cerr << "\to: \"" << Expected << '\"' << endl << "\tx: \"" << Received << '\"' << endl; } }
#define CASE(N) {cerr << "Test Case #" << N << "..." << flush; start_time=clock();
#define END	 verify_case(_, FixedDiceGameDiv1().getExpectation(a, b, c, d));}
int main(){

CASE(0)
	int a = 1; 
	int b = 2; 
	int c = 1; 
	int d = 5; 
	double _ = 2.0; 
END
CASE(1)
	int a = 3; 
	int b = 1; 
	int c = 1; 
	int d = 3; 
	double _ = 3.0; 
END
CASE(2)
	int a = 1; 
	int b = 5; 
	int c = 1; 
	int d = 1; 
	double _ = 3.4999999999999996; 
END
CASE(3)
	int a = 2; 
	int b = 6; 
	int c = 50; 
	int d = 30; 
	double _ = -1.0; 
END
CASE(4)
	int a = 50; 
	int b = 11; 
	int c = 50; 
	int d = 50; 
	double _ = 369.8865999182022; 
END
/*
CASE(5)
	int a = ; 
	int b = ; 
	int c = ; 
	int d = ; 
	double _ = ; 
END
CASE(6)
	int a = ; 
	int b = ; 
	int c = ; 
	int d = ; 
	double _ = ; 
END
*/
}
// END CUT HERE
