function main(input[0]) {
    var correct = 0;

    for(var i=0; i<parseInt((input.length)/2); i++) {
        if(input[i] != input[input.length-i-1] ){
            correct+=1;
        }
    }
    console.log(correct);
}

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