s = gets.chomp.split("")

mid = (s.length-1) / 2

front = Array.new(mid)
back = Array.new(mid)

for i in 0..mid do
    front[i] = s[i]
end

for i in mid+1..s.length-1 do
    back[s.length - 1 - i] = s[i]
end

if(front.length != back.length)
    front.pop
end

tigau = 0

for i in 0..front.length-1 do
    if(front[i] != back[i])
        tigau += 1
    end
end

puts tigau