d = [0,1,0,2,1,0,1,3,2,1,2,1]
#print(d)
n = len(d)
 
d_l = [0 for _ in range(n)]  # max heights from left to right at the given position d[i]
max_l = 0
for i in range(n):
	max_l = max(max_l, d[i])
	d_l[i] = max_l
#print(d_l)
 
d_r = [0 for _ in range(n)]  # max heights from right to left at the given position d[i]
max_r = 0
for i in range(n-1, -1, -1):
	max_r = max(max_r, d[i])
	d_r[i] = max_r
#print(d_r)
 
ans = 0
for i in range(n):
	ans += min(d_l[i], d_r[i]) - d[i]
 
print(ans)

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