"use strict";
main(require("fs").readFileSync("/dev/stdin", "UTF-8"));

function main(arg) {
    arg=arg.split("\n")[0].split("")
    let n = arg.length;
    let ijougen = Math.floor(n/2);
    let kekka = 0;
    let honbun = arg;
    for(let i=0;i<ijougen;i++){
    	if(honbun[i]!=honbun[n-i-1])
        {kekka += 1;
        }
    }
    console.log(kekka);
}

