"use strict"
function Main(input) {
  input = input.trim().split('');
  const len = input.length;
  
  let ret = 0;
  if(len%2 === 0){
    let pre = input.slice(0, len/2);
    let bak = input.slice(len/2, len).reverse();
    let max = pre.length;
    
    for(let i=0; i<max; i++){
      if(pre[i] !== bak[i]) ret += 1;
    }
  } else {
    let pre = input.slice(0, len/2);
    let bak = input.slice(len/2+1, len).reverse();
    let max = pre.length;

    for(let i=0; i<max; i++){
      if(pre[i] !== bak[i]) ret += 1;
    }
  }

  console.log(ret);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));