def ascan; gets.split.map(&:to_i); end

@memo = {}
def dfs(n,last = nil)
    return 1 if n == 0
    return 0 if n < 0
    return dfs(n-1, 1) if last == 2
    return @memo[n] if @memo[n]
    return @memo[n] = dfs(n-2, 2) + dfs(n-1, 1)
end

p dfs(gets.to_i)