import std.stdio, std.string, std.conv, std.array, std.algorithm;
import std.uni, std.range, std.math, std.container, std.datetime;
import core.bitop, std.typetuple, std.typecons;

immutable long MOD = 1_000_000_007;
alias tie = TypeTuple;

void main(){
    int vl, vr;
    readVars(vl, vr);
    auto d = readln.chomp.to!int;
    auto w = readln.chomp.to!int;

    auto ans = 1. * w * d / (vl + vr);
    writefln("%.7f", ans);
}


void readVars(T...)(auto ref T args){
    auto line = readln.split;
    foreach(ref arg ; args){
        arg = line.front.to!(typeof(arg));
        line.popFront;
    }
    if(!line.empty){
        throw new Exception("args num < input num");
    }
}