function main(arg) {
    var n = arg.length;
    var ijougen = Math.floor(n/2);
    var kekka = 0;
    var honbun = arg.replace(/\s/g,"");
    for(i=0;i<ijougen;i++){
    	if(honbun.charAt(i)!=honbun.charAt(n-i-1)){kekka += 1;}
    }
    console.log(kekka);
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));