function main(arg) {
    var mojiretu = arg;
    var kosuu = 0;
	for(i=0;i<mojiretu.length/2;i++){
		if(mojiretu.charAt(i)!==mojiretu.charAt(mojiretu.length-i-1)){
			kosuu += 1;
		}
	}
	console.log(kosuu);
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));