import sys
input=sys.stdin.readline
MOD=998244353
def main():
    N=int(input())
    a=list(map(int,input().split()))
    pw2=[1]*(N+1)
    for i in range(N):
        pw2[i+1]=(pw2[i]*2)%MOD
    ans=0
    for i in range(N):
        if abs(a[i])==2:
            prod=0
            cl=[pw2[max(0,i-1)],0]
            for j in range(i-1,-1,-1):
                if abs(a[j])==1:
                    if a[j]==-1:
                        prod^=1
                    cl[prod]=(cl[prod]+pw2[max(0,j-1)])%MOD
                else:
                    break
            prod=0
            cr=[pw2[max(0,N-2-i)],0]
            for j in range(i+1,N):
                if abs(a[j])==1:
                    if a[j]==-1:
                        prod^=1
                    cr[prod]=(cr[prod]+pw2[max(0,N-2-j)])%MOD
                else:
                    break
            if a[i]==2:
                ans=(ans+cl[0]*cr[1]+cl[1]*cr[0])%MOD
            else:
                ans=(ans+cl[0]*cr[0]+cl[1]*cr[1])%MOD
    ans*=pow(pow(2,N-1,MOD),MOD-2,MOD)
    ans%=MOD
    print(ans)
if __name__=='__main__':
    main()