def main():
    vl, vr = input().split()
    vl = int(vl)
    vr = int(vr)
    d = int(input())
    w = int(input())
    print(d*w/(vr+vl))


if __name__ == "__main__":
    import sys
    import os
    if len(sys.argv) > 1:
        if sys.argv[1] == "-d":
            filename = "input1.txt"
            fd = os.open(filename, os.O_RDONLY)
            os.dup2(fd, sys.stdin.fileno())
            main()
    else:
        main()