let posP:[Int] = readLine()!.split(separator: " ").map{Int($0)!}
let posQ:[Int] = readLine()!.split(separator: " ").map{Int($0)!}
let distance = abs(posP[0] - posQ[0] + posP[1] + posQ[1])
let time:Double = Double(distance) / 2.0
print(time)