"use strict";

function Main(input) {
  let num = input.length;
  let ans = 0;
  for(let i = 0; i < Math.floor(num/2); i++){
  	if(input[i] != input[num - i - 1]){
    	ans++;
    }
  }
  console.log(ans);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
