def count(height):
    N = len(height)
    left = height[:N // 2]
    right = height[N // 2:][::-1]
    bullets = 0
    for i in range(N // 2):
        if right[i] > left[i]:
            bullets += right[i] - left[i]
            right[i] = left[i]
    return bullets


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

minimum = count(heights)
print(minimum)