#include <stdio.h>

int main(void) {
	int vl, vr;
	int d;
	int w;
	scanf("%d%d", &vl, &vr);
	scanf("%d", &d);
	scanf("%d", &w);
	printf("%f\n", (double)d / ( vl + vr) * w);
	
	return 0;
}