"use strict";

const main = arg => {
  const a = arg.trim();
  let num_diffrent = 0;
  for(let i=0;i<a.length/2;i++){
    if(a[i]!==a[a.length-i-1]){
      num_diffrent++;
    }
  }
  return num_diffrent;
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));