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

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

