let s=readLine()!
let n=s.count
var S=Array(s)
var count=0
if n%2==0{
  for i in 0...n/2-1{
    if S[i] != S[-1-i]{
      count+=1
    }
  }
}else{
   for i in 0...(n-1)/2-1{
    if S[i] != S[-1-i]{
      count+=1
    }
  }
}
print(count) 