function main(agr) {
    var input = agr[0];
    var count = 0;
    var n = input.length;
    for(var i = 0; i < n ; i++) {
        if(input[i] !== input[n-1-i]) {
            count ++;
        }
    }

    console.log(Math.floor(count/2));
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));