import Foundation

let S = readLine()!
let T = readLine()!
let nums = S.count
 
var min = 0
 
for i in 0...nums - 1 {
    if S[S.index(S.startIndex, offsetBy: i)] != T[T.index(T.startIndex, offsetBy: i)] {
        min += 1
    }
}
 
print(min)