import Foundation

extension String {
    func split(separator: String) -> [Int] {
        return self.componentsSeparatedByString(separator).map { Int($0)! }
    }
}

let input1 = readLine()!.split(" ")
let H = input1[0]
let W = input1[1]
let input2 = readLine()!.split(" ")
let h = input2[0]
let w = input2[1]

print((H-h)*(W-w))