#395 17 forever
import math
N=int(input())
if N<15:
    print(-1)
else:
    i=8
    nd=0
    while nd==0 and i<=math.sqrt(N):
        if N%i==0:
            nd=i
        i+=1 
    if nd>0:
        print(nd)
    else:
        print(N-7)