import sys
from random import seed, randrange

def modpow(base, power, mod):
    result = 1
    while power > 0:
        if power & 1 == 1:
            result = (result * base) % mod
        base = (base * base) % mod
        power >>= 1
    return result

def isPrime(n):
    if n % 2 == 0:
        return False
    
    def MR_core(d, a, s = 1):
        if s == 1:
            return MR_core(d >> 1, a, 2) == n
        elif s == 2:
            if d & 1:
                m = MR_core(d >> 1, a, 3)
                s = m * m % n * a % n
                return n if s == n - 1 or s == 1 else s
            else:
                m = MR_core(d >> 1, a, 2)
                if m == n:
                    return n
                else:
                    s = m * m % n
                    return n if s == n - 1 else s
        elif s == 3:
            if d == 0:
                return 1
            else:
                m = MR_core(d >> 1, a, 3)
                if d & 1:
                    return m * m % n * a % n
                else:
                    return m * m % n
    
    k = 3
    return all(MR_core(n - 1, randrange(2, n)) for i in range(k))

n = input()
umekomi = [ 1, 1, 2, 3, 3, 5, 3, 7, 7, 7, \
            5,11,11,13, 7, 7, 7, 8, 8,19, \
           19, 7, 7,23,23, 8, 8, 8, 7, 8]
if n <= 30:
    print umekomi[n-1]
    sys.exit()

m = 8
while True:
    if isPrime(m+1) == False:
        if n%m != 1 or isPrime(n-m) == False:
            print m
            sys.exit()
    m += 2