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)))345. Reverse Vowels of a String | 628. Maximum Product of Three Numbers |
1526A - Mean Inequality | 1526B - I Hate 1111 |
1881. Maximum Value after Insertion | 237. Delete Node in a Linked List |
27. Remove Element | 39. Combination Sum |
378. Kth Smallest Element in a Sorted Matrix | 162. Find Peak Element |
1529A - Eshag Loves Big Arrays | 19. Remove Nth Node From End of List |
925. Long Pressed Name | 1051. Height Checker |
695. Max Area of Island | 402. Remove K Digits |
97. Interleaving String | 543. Diameter of Binary Tree |
124. Binary Tree Maximum Path Sum | 1465. Maximum Area of a Piece of Cake After Horizontal and Vertical Cuts |
501A - Contest | 160A- Twins |
752. Open the Lock | 1535A - Fair Playoff |
1538F - Interesting Function | 1920. Build Array from Permutation |
494. Target Sum | 797. All Paths From Source to Target |
1547B - Alphabetical Strings | 1550A - Find The Array |