def Question(x):
    print(f'? {x}')
    return int(input())

def Answer(x):
    print(f'! {x}')
    exit()

N = int(input())
S = []
for i in range(2,N+1):
    flg = True
    for k in S:
        if i % k == 0:
            flg = False
            break
    if flg:
        S.append(i)
P = [1] * (N+1)
for i in S:
    if Question(i) >= 2:
        for j in range(2,N+1):
            if j % i != 0:
                P[j] = 0
    else:
        for j in range(i,N+1,i):
            P[j] = 0
for i in range(N,-1,-1):
    if P[i]:
        Answer(i)