#include <cstdio>
#include <cmath>

typedef long long   signed int LL;
typedef long long unsigned int LU;

#define incID(i, l, r) for(int i = (l)    ; i <  (r); i++)
#define incII(i, l, r) for(int i = (l)    ; i <= (r); i++)
#define decID(i, l, r) for(int i = (r) - 1; i >= (l); i--)
#define decII(i, l, r) for(int i = (r)    ; i >= (l); i--)
#define inc( i, n) incID(i, 0, n)
#define inc1(i, n) incII(i, 1, n)
#define dec( i, n) decID(i, 0, n)
#define dec1(i, n) decII(i, 1, n)

#define inII(v, l, r) ((l) <= (v) && (v) <= (r))
#define inID(v, l, r) ((l) <= (v) && (v) <  (r))

template<typename T> void swap(T & x, T & y) { T t = x; x = y; y = t; return; }
template<typename T> T abs(T x) { return (0 <= x ? x : -x); }
template<typename T> T max(T a, T b) { return (b <= a ? a : b); }
template<typename T> T min(T a, T b) { return (a <= b ? a : b); }
template<typename T> bool setmin(T & a, T b) { if(a <= b) { return false; } else { a = b; return true; } }
template<typename T> bool setmax(T & a, T b) { if(b <= a) { return false; } else { a = b; return true; } }
template<typename T> T gcd(T a, T b) { return (b == 0 ? a : gcd(b, a % b)); }
template<typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }

// ---- ----

int l, r, d, w;

int main() {
	scanf("%d%d%d%d", &l, &r, &d, &w);
	
	printf("%f\n", 1.0 * d * w / (l + r));
	
	return 0;
}