import sys


def input(): return sys.stdin.readline().rstrip()


def main():
    N = int(input())
    *A, = map(int, input().split())

    def pyramid(M):
        return list(range(1, M)) + list(range(M, 0, -1))

    def dist(a, b):
        u, d = 0, 0
        for i in range(max(len(a), len(b))):
            A = a[i] if i < len(a) else 0
            B = b[i] if i < len(b) else 0
            if A > B:
                u += A-B
            else:
                d += B-A
        return max(u, d) if u <= d else float('inf')

    ans = min(dist(pyramid(i), A) for i in range(1, N+1))

    print(ans)


if __name__ == '__main__':
    main()