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

have = []
ans = 0
for i in range(n):
    if b[i] > 1:
        have.append((i, b[i]))
    if b[i] == 0 and have != []:
        g, p = have.pop()
        b[i] += 1
        b[g] -= 1
        ans += i-g
        if p-1 > 1:
            have.append((g, p-1))

have = []
for i in range(n)[::-1]:
    if b[i] > 1:
        have.append((i, b[i]))
    if b[i] == 0 and have != []:
        g, p = have.pop()
        b[i] += 1
        b[g] -= 1
        ans += g-i
        if p-1 > 1:
            have.append((g, p-1))

print(ans)