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a1 = [0,0]
a2 = [1,1]
n = int(raw_input())
s = [int(i) for i in raw_input().split()]
if n==0:
    print 0
    exit()
for i in s:
    if i == 1:
        if a1[0] > 0:
            a1[1] = (a1[1]+a1[0])%1000000007
            a1[0] = (a1[0]+1)%1000000007
            if len(a2) == 2:
                a1.append(0)
                a2.append(2)
        else:
            a1[0] = (a1[0]+1)%1000000007
    elif i in a2:
        j = a2.index(i)
        a1[j] = (a1[j]+a1[j-1])%1000000007
        if j==len(a2)-1:
            a2.append(a2[j-1]+a2[j])
            a1.append(0)
num = 0
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