import sys

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline

def from_read():
    return [int(x) for x in read().splitlines()]

def from_readline():
    return [int(x) for x in readline().split()]

def read_matrix(N, M):
    matrix = [None for i in range(N)]
    for i in range(N):
        matrix[i] = from_readline()
    return matrix

def solve(xa, ya, xb, yb):
    return (ya*xb + yb*xa) / (xa + xb)

def main():
    xa, ya = from_readline()
    xb, yb = from_readline()
    print(solve(xa, ya, xb, yb))

if __name__ == "__main__":
    main()