`use strict`

function Main(input) {
  const a = input.trim().split('')
  const N = a.length

  if(N === 1) {
    console.log(0)
    return
  }

  let c = 0
  for(let i = 0; i < Math.floor(N/2); ++i) {
    const t = a[i]
    const l = a[N - 1 - i]
    if (t !== l) c++
  }
  console.log(c)
  
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));