#!/bin/bash
':' /*
docker run --rm -it -v $(cd $(dirname $0) && pwd):/vo:ro node:5.2-slim \
  /bin/bash -c "node /vo/$(basename $0) < /vo/g.inp"; exit $?
exit 0
*/

"use strict";

let main = (input) => {
    let j = input.length;
    let lc = Math.ceil(j/2);
    let cnt = 0;
    for(let i=0; i<lc; i++) {
        j--;
        if (input.substring(i,i+1)!=input.substring(j,j+1)) cnt+=1;
    }
    return cnt;
}
console.log(main(require('fs').readFileSync("/dev/stdin", "utf-8").trim()));