func main()
	var n: int :: cui@inputInt()
	var isPrime: []bool :: sieve(n)
	var dp: []int :: #[n + 1]int
	for p(2, n)
		if(isPrime[p])
			for i(n, 2, -1)
				if(dp[i] <> 0)
					if(i + p <= n)
						do dp[i + p] :: [dp[i + p], dp[i] + 1].max()
					end if
				end if
			end for
			do dp[p] :: [dp[p], 1].max()
		end if
	end for
	var m: int :: dp[n] = 0 ?(-1, dp[n])
	do cui@print("\{m}\n")
	
	func sieve(n: int): []bool
		if(n < 2)
			ret[false].repeat(n + 1)
		end if
		var isPrime: []bool :: [true].repeat(n + 1)
		do isPrime[0] :: false
		do isPrime[1] :: false
		for i(2, n)
			if(isPrime[i])
				var j: int :: i * i
				while(j <= n)
					do isPrime[j] :: false
					do j :+ i
				end while
			end if
		end for
		ret isPrime
	end func
end func