import Foundation

func readInts() -> [Int] {
    return readLine()!.split(separator: " ").map { Int($0)! }
}

let S=readLine()!

var diff=0

var i=0

for _ in S{
  let c1=S[S.index(S.startIndex,offsetBy:i)]
  let c2=S[S.index(S.endIndex,offsetBy:-i-1)]
  if c1 != c2{diff+=1}
  i+=1
}

print(diff/2)