import core.bitop, std.bitmanip;
import core.checkedint;
import std.algorithm, std.functional, std.meta;
import std.array, std.container;
import std.bigint;
import std.conv;
import std.math, std.numeric;
import std.range, std.range.interfaces;
import std.stdio, std.string;
import std.typecons;

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