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);
}
 
/* var input = `8
hmhmnknk uuuuuuuu`;
Main(input);
 */ 
Main(require("fs").readFileSync("/dev/stdin", "utf8"));