function Main(input) {
  input = input.split("");
  var cou=0;
  for(var i in input){
    if(i>=input.length-i-1)break;
    if(input[i]==input[input.length-i-1]){

    }else{
      cou++;
    }
  }
  console.log(cou);
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));