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

s = sum(a)
t = int(s**0.5)
b = [i+1 for i in range(t)] + [i for i in range(t-1, 0, -1)]

c = 0
for i in range(min(len(a), len(b))):
    c += min(a[i], b[i])

d = s - c
print(d)