"use strict";

function main(input) {
  input = input.trim().split("");
  let ans = 0;
  for (let i = 0; i < Math.floor(input.length / 2); i++) {
    if (input[i] !== input[input.length - (i + 1)]) {
      ans++;
    }
  }
  console.log(ans);
}

main(require("fs").readFileSync("/dev/stdin", "utf-8"));
