n = int(input())
b = list(map(int, input().split()))
ans = 0
current_target = 0
for i in range(n):
    cnt = b[i]
    if cnt == 0:
        continue
    start = current_target
    end = current_target + cnt - 1
    if i >= end:
        total = cnt * i - (start + end) * cnt // 2
    elif i <= start:
        total = (start + end) * cnt // 2 - cnt * i
    else:
        m_left = i - start + 1
        sum_left = i * m_left - (start + i) * m_left // 2
        m_right = end - i
        sum_right = m_right * (m_right + 1) // 2
        total = sum_left + sum_right
    ans += total
    current_target += cnt
print(ans)