#include <iostream>
#include <stack>
#include <algorithm>

using ui = unsigned long int;

struct column {
	ui house = 0, leftHeight = 0;
};

int main() {
	size_t n;
	std::cin >> n;
	std::stack<column> s;
	column current;
	for (size_t i = 0; i < n; ++i) {
		std::cin >> current.house;
		current.leftHeight = std::max(current.house, current.leftHeight);
		s.push(current);
		++(current.leftHeight);
	}
	ui sand = 0, rightHeight = 0;
	while (!s.empty()) {
		current = s.top();
		s.pop();
		rightHeight = std::max(current.house, rightHeight);
		sand += (std::min(rightHeight, current.leftHeight) - current.house) * 4 + (rightHeight == current.leftHeight ? 1 : 2);
		++rightHeight;
	}
	std::cout << sand;
}