n = int(input())
a = list(map(int, input().split()))

ans = 0

for i in range(n):
    now = a[i]
    ans += now - i - 1
    a[i] = i + 1
    for j in range(i + 1, n):
        if a[j] < now:
            a[j] += 1

print(ans)