#include <cstdio>

using namespace std;
//
//FILE* in = freopen("date.in", "r", stdin);
//FILE* out = freopen("date.out", "w", stdout);

int t1, k1, t2, k2;

int main()
{
	double t3;
	scanf("%d %d %d %d", &t1, &k1, &t2, &k2);
	t3 = (double)(t1 * t2) / (double)(k1  * t2 + k2  * t1);
	printf("%lf\n", t3);
	return 0;
}