N = int(input())
A = [int(x) for x in input().split()]

result = 0
clusters = [0] + [i for i in range(1, N) if A[i - 1] + 1 != A[i]]
for i in reversed(clusters):
    if i == 0:
        target = 1
    else:
        target = A[i - 1] + 1
    result += A[i] - target
print(result)