import std.stdio, std.string, std.algorithm, std.array, std.range, std.conv,
    std.typecons, std.math, std.container, std.format;

void main(string[] args)
{
    auto s = readln.strip();
    long cnt = 0;
    foreach (i; 0 .. s.length / 2)
    {
        if (s[i] != s[s.length - i - 1])
        {
            cnt++;
        }
    }
    writeln(cnt);
}
