N = int(input())
B = list(map(int, input().split()))
MOD = 998244353

ct = 0
for i in range(1, N-1):
    if B[i] == -1:
        ct += 1

a = B[0]
b = B[-1]
if a == b:
    if a >= 0:
        print(0)
        exit()
    ct += 1
print(pow(2, ct, MOD))