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

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

print(ans)