function main (input){
	const inputs = input.split('');
	const length = inputs.length;
	let num = 0;
	for (let i = length; i--;){
		if(inputs[i] !== inputs[length - 1 -i]) {
			num++;
		}
	}
	console.log(num / 2);
}

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