import java.io.*;
import java.util.*;

public class Main{
    public static Scanner sc = new Scanner(System.in);
    public static int n;
    public static boolean isPrimes(int n) {
        for(int i = 2; i <= n / i; i++)
            if(n % i == 0) return false;
        return true;
    }
    public static int getMaxPrimes(int n) {
        int res = 0;
        int i = 2;
        while(n > 1) {
            if(n % i == 0) {
                res = Math.max(res, i);
                while(n % i == 0) n /= i;
            }else i++;
        }
        return res;
    }
    public static void main(String[] args) {
        n = sc.nextInt();
        if(isPrimes(n)) {
            System.out.println("-1");
            System.exit(0);
        }
        int pos = getMaxPrimes(n);
        int res = Integer.MAX_VALUE;
        for(int m = n - pos + 1; m <= n; m++) {
            if(isPrimes(m)) continue;
            int p = getMaxPrimes(m);
            int x = m - p + 1;
            res = Math.min(res, x);
        }
        System.out.println(res);
    }
}