import Foundation

func readInt() -> Int {
    return Int(readLine()!)!
}

func readInts() -> [Int] {
    return readLine()!.split(separator: " ").map { Int($0)! }
}

// ABC178B
let input = readInts()
let a = input[0]
let b = input[1]
let c = input[2]
let d = input[3]

print(max(a * c, a * d, b * c, b * d))
