def calc(X):
    v = S
    for i in range(min(len(X), len(A))):
        v -= min(X[i], A[i])
    return v

N = int(input())
A = list(map(int, input().split()))
S = sum(A)
ans = 10 ** 18
for i in range(1, 200):
    L = list(range(1, i + 1))
    for j in range(i - 1, -1, -1):
        L.append(j)
    if sum(L) > S:
        break
    ans = min(ans, calc(L))
print(ans)