package main

import (
	"fmt"
	"math/big"
	"os"
)

type UnionFind struct {
	n  int
	ig []int
	gi [][]int
}

func MakeUF(n int) *UnionFind {
	uf := new(UnionFind)
	uf.n = n
	uf.ig = make([]int, n)
	uf.gi = make([][]int, n)
	for i := 0; i < n; i++ {
		uf.ig[i] = i
		uf.gi[i] = []int{i}
	}
	return uf
}

func (uf *UnionFind) merge(a, b int) {
	a = uf.ig[a]
	b = uf.ig[b]
	if a == b {
		return
	}
	if len(uf.gi[a]) < len(uf.gi[b]) {
		tmp := a
		a = b
		b = tmp
	}
	for _, c := range uf.gi[b] {

		uf.ig[c] = a
	}
	uf.gi[a] = append(uf.gi[a], uf.gi[b]...)
}

func (uf *UnionFind) same(a, b int) bool {
	return uf.ig[a] == uf.ig[b]
}

const PS = 50000

var pr [PS]bool

func init() {
	for i := 2; i < PS; i++ {
		pr[i] = true
	}
	for i := 2; i < PS; i++ {
		if pr[i] == false {
			continue
		}
		for j := 2 * i; j < PS; j += i {
			pr[j] = false
		}
	}
}

func solve1(n int) int {
	for i := 2; i <= n; i++ {
		uf := MakeUF(n + 1)
		for j := 1; j <= n; j++ {
			if j%i != 1 {
				if !pr[j] && !pr[j-1] {
					uf.merge(j, j-1)
				}
			}
			if i < j {
				if !pr[j] && !pr[j-i] {
					uf.merge(j, j-i)
				}
			}
		}
		if uf.same(1, n) {
			return i
		}
	}
	os.Exit(1)
	return -1
}

func solve2(n *big.Int) int {
	for i := 2; i < 1000; i++ {
		const B = 9000
		{
			uf := MakeUF(B + 1)
			for j := 1; j <= B; j++ {
				if j%i != 1 {
					if !pr[j] && !pr[j-1] {
						uf.merge(j, j-1)
					}
				}
				if i < j {
					if !pr[j] && !pr[j-i] {
						uf.merge(j, j-i)
					}
				}
			}
			if !uf.same(1, B/i*i) {
				continue
			}
		}
		{
			uf := MakeUF(B + 1)
			ii := big.NewInt(int64(i))
			bpr := make([]bool, B+1)
			for jj := 0; jj <= B; jj++ {
				j := new(big.Int)
				j.Set(n).Sub(j, big.NewInt(int64(B-jj)))
				bpr[jj] = j.ProbablyPrime(100)
			}
			for jj := 1; jj <= B; jj++ {
				j := new(big.Int)
				j.Set(n).Sub(j, big.NewInt(int64(B-jj)))
				k := new(big.Int)
				k.Set(j)
				if k.Mod(k, ii).Cmp(big.NewInt(1)) != 0 {
					if !bpr[jj] && !bpr[jj-1] {
						uf.merge(jj, jj-1)
					}
				}
				if i <= jj {
					if !bpr[jj] && !bpr[jj-i] {
						uf.merge(jj, jj-i)
					}
				}
			}
			nn := new(big.Int)
			nn.Set(n)
			if uf.same(B-int(nn.Mod(nn, big.NewInt(int64(i))).Int64()), B) {
				return i
			}
		}
	}
	os.Exit(1)
	return -1
}

func main() {
	b := new(big.Int)
	fmt.Scan(b)
	if b.Cmp(big.NewInt(20000)) == -1 {
		fmt.Println(solve1(int(b.Int64())))
	} else {
		fmt.Println(solve2(b))
	}
}