Choose and divide UVA - 10375  唯一分解定理

//唯一分解定理
#include<iostream>
#include<cmath>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<vector>
#include<algorithm>
#pragma warning(disable:4996)
#define me(s)  memset(s,0,sizeof(s))
#define _for(i,a,b) for(int i=(a);i<(b);++i)
#define _rep(i,a,b) for(int i=(a);i<=(b);++i)
using namespace std;
typedef pair <int, int> pii;
typedef long long ll;
typedef unsigned long long ull;
const int inf = 0x3f3f3f3f;
const int dr[] = { 0, -1, 0, 1, -1, -1, 1, 1 };
const int dc[] = { -1, 0, 1, 0, -1, 1, -1, 1 };
const int MOD = 1e9 + 7;
const double pi = acos(-1.0);
const double eps = 1e-15;
const int maxn = 10000;
vector<int> primes;
int e[maxn];

void add_integer(int n, int d) {
	for (int i = 0; i < primes.size(); i++) {
		while (n % primes[i] == 0) {
			n /= primes[i];
			e[i] += d;
		}
		if (n == 1) break;
	}
}

void add_factorial(int n, int d) {
	for (int i = 1; i <= n; i++)
		add_integer(i, d);
}

bool is_prime(int n) {
	int m = floor(sqrt(n) + 0.5);
	for (int a = 2; a <= m; a++)
		if (n % a == 0) return false;
	return true;
}

int main() {
	for (int i = 2; i <= 10000; i++)
		if (is_prime(i)) primes.push_back(i);
	int p, q, r, s;
	while (cin >> p >> q >> r >> s) {
		memset(e, 0, sizeof(e));
		add_factorial(p, 1);
		add_factorial(q, -1);
		add_factorial(p - q, -1);
		add_factorial(r, -1);
		add_factorial(s, 1);
		add_factorial(r - s, 1);
		double ans = 1;
		for (int i = 0; i < primes.size(); i++)
			ans *= pow(primes[i], e[i]);
		printf("%.5lf\n", ans);
	}
	return 0;
}