n=int(input())
lis=input().split()
siz = [0] * (n+1)
for i in range(n):
lis[i] = int(lis[i])
if lis[i] > 0:
siz[lis[i]] += 1
r = 0
for i in range(n+1):
r += 2**siz[i] - 1
r = 2**(n-1) - 1 - r
print(int(r % (10**9 + 7)))1607A - Linear Keyboard | EQUALCOIN Equal Coins |
XOREQN Xor Equation | MAKEPAL Weird Palindrome Making |
HILLSEQ Hill Sequence | MAXBRIDGE Maximise the bridges |
WLDRPL Wildcard Replacement | 1221. Split a String in Balanced Strings |
1002. Find Common Characters | 1602A - Two Subsequences |
1555A - PizzaForces | 1607B - Odd Grasshopper |
1084A - The Fair Nut and Elevator | 1440B - Sum of Medians |
1032A - Kitchen Utensils | 1501B - Napoleon Cake |
1584B - Coloring Rectangles | 1562B - Scenes From a Memory |
1521A - Nastia and Nearly Good Numbers | 208. Implement Trie |
1605B - Reverse Sort | 1607C - Minimum Extraction |
1604B - XOR Specia-LIS-t | 1606B - Update Files |
1598B - Groups | 1602B - Divine Array |
1594B - Special Numbers | 1614A - Divan and a Store |
2085. Count Common Words With One Occurrence | 2089. Find Target Indices After Sorting Array |