MOD = 10**9 + 7
m = input()
H = map(int, raw_input().split())
if H[0] == 0:
    print 1
else:
    cnt = m - (sum(H) + len(H)-1) + len(H)
    if cnt < len(H):
        print "NA"
        exit(0)
    fact = [1]*(m+1)
    rev = [1]*(m+1)
    for i in xrange(1, m+1):
        fact[i] = (fact[i-1]*i) % MOD
    print (fact[cnt]*pow(fact[len(H)], MOD-2, MOD)*pow(fact[cnt-len(H)], MOD-2, MOD)) % MOD