require 'pp'

n = gets.chomp!.to_i

def f(n)
    if n == 1 || n == 0
        1
    else
        n * f(n-2)
    end
end

if n % 2 != 0
    puts 0
    exit 0
end

count = 0

# 10 の倍数の数
tmp = 5
while n >= tmp
    count += (n / tmp) /2
    tmp *= 5
end

puts count


