let aPos = readLine()!.split(separator: " ").map{Int($0)!}
let bPos = readLine()!.split(separator: " ").map{Int($0)!}

let dist:Int = abs(aPos[0] - bPos[0]) + abs(aPos[1] - bPos[1])

if(dist%2 == 0) {
    print(dist / 2)
} else {
    print(Double(dist) / 2.0)
}