function main(arg) {
	arg = arg.trim();
  	var long = arg.length,
        c = Math.round(long/2),
  		out = 0;
	for (var i = 0; i < c; i ++) {
    	var stword = arg[i],
           	lsword = arg[long - i -1];
      	if (stword != lsword) {
        	out++;
        }
    }
  console.log(out)
  	
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));