def calc_nb(h):
    return h**2

N = int(input())
A = [int(i) for i in input().split()]
num_b = sum(A)

btm = 0
top = 1000

while top - btm > 1:
    mid = (top + btm) // 2

    if num_b >= calc_nb(mid):
        btm = mid
    else:
        top = mid

h = btm
trash = num_b - calc_nb(h)
ans = trash

if 2*h - 1 > N:
    A += [0] * (2*h - 1 - N)

for i in range(h - 1):
    ans += max(0, (i + 1) - A[i])

for i in range(h - 1):
    ans += max(0, (i + 1) - A[2*h - 2 - i])

ans += max(0, h - A[h - 1])

print(ans)