import Foundation

let n = readLine()!.split(separator: " ").map{Int($0)!}
var t = readLine()!
var b = readLine()!
var ta = t.count - t.replacingOccurrences(of: "A", with: "").count
var tb = t.count - ta
var ba = b.count - b.replacingOccurrences(of: "A", with: "").count
var bb = b.count - ba

print([ta,ba].min()! + [tb,bb].min()!)