def greedy(A):
    while len(A)>1:
        B=[]
        for i in range(len(A)-1):
            B.append(A[i]^A[i+1])
        A=B
    return A[0]

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

Mod=998244353
Q=B[1:-1].count(-1); Q_exp=pow(2,Q,Mod)

X=0
for a in [0,1]:
    for b in [0,1]:
        if B[0] in {-1,a} and B[-1] in {-1,b} and a!=b:
            X+=Q_exp

print(X%Mod)