'use strict'
function Main(input) {

  const S = input.split("\n")[0];
  const Sr = S.split("").reverse().join("");
  const num = Math.floor(S.length / 2);

  let count = 0;

  for (let i = 0; i < num; i++) {
    if (S[i] !== Sr[i]) {
      count += 1;
    }
  }
  console.log(count);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));
