function main(input){

  var leng = input.length;
  var count = 0;
  var count_max = leng/2;
  var out = 0 ;

  while(count < count_max){
    if(input[count]!=input[leng-count-1]){
      out++;
    }
    count++;
  }
  console.log(out);
}

main(require("fs").readFileSync("/dev/stdin", "utf8"));