"use strict";

function Main(input) {
  let num = input.trim().length;
  let Rinput = input.trim().split("").reverse().join("");
  let ans = 0;

  for(let i = 0; i < num; i++){
  	if(input[i] != Rinput[i]){
    	ans++;
    }
  }
  console.log(Math.ceil(ans/2));
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
