import Foundation

func readStr() -> [String] {
    readLine()!.map { String($0) }
}

func main(){
    let s = readStr()
    let t = readStr()
    var ans = 0
    
    for i in 0..<t.count {
        if s[i] != t[i] {
            ans += 1
        }
    }
    print(ans)
}
main()

