#!/usr/bin/env python3

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

leftHeights = []
leftHeight = 0
for house in houses:
	leftHeight = max(house, leftHeight)
	leftHeights.append(leftHeight)
	leftHeight += 1

sand = 0
rightHeight = 0
for house, leftHeight in zip(reversed(houses), reversed(leftHeights)):
	rightHeight = max(house, rightHeight)
	sand += (min(leftHeight, rightHeight) - house) * 4 + (1 if leftHeight == rightHeight else 2);
	rightHeight += 1

print(sand)