import std.algorithm;
import std.array;
import std.conv;
import std.math;
import std.range;
import std.stdio;
import std.string;

void main() {
  auto s = readln.strip;
  int t;
  for (size_t i = 0, j = s.length - 1; i < j; i++,j--) {
    if (s[i] != s[j]) ++t;
  }
  writeln (t);
}

