n = int(input())
a_list = list(map(int, input().split()))
block_sum = sum(a_list)

required = 1
count = 0
for i in range(1, 200):
    tmp = required + 2 * i + 1
    if tmp > block_sum:
        count = i
        break
    required = tmp

result = 0
for i in range(1, n + 1):
    if i >= count * 2:
        result += a_list[i-1]
    elif i > count:
        result += max(a_list[i-1] - (count * 2 - i), 0)
    else:
        result += max(a_list[i-1] - i, 0)

print(result)