require"prime"
N=20000
dp=Array.new(N+1)
dp[0]=0
temp=0
Prime.each(N/2){|p|
	(N-p).downto(0){|k|
		next unless dp[k]
		temp=k+p
		unless dp[temp]
			dp[temp]=dp[k]+1
		else
			dp[temp]=dp[k]+1 if dp[k]+1>dp[temp]
		end 
	}
}

p dp[gets.to_i]||-1