#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main() {
	size_t n;
	cin >> n;
	vector<int> v(n);
	for (size_t i = 0; i < n; ++i) {
		cin >> v[i];
	}
	int result = 0;
	while (!v.empty()) {
		vector<int> gain(v.size());
		for (size_t i = 0; i < v.size(); ++i) {
			gain[i] = v[i] 
				- ((i > 0) ? v[i - 1] : 0)
				- ((i < v.size() - 1) ? v[i + 1] : 0);
		}
		size_t maxIndex = distance(gain.begin(), max_element(gain.begin(), gain.end()));
		result += v[maxIndex];
		if (maxIndex < gain.size() - 1) {
			gain.erase(gain.begin() + maxIndex + 1);
			v.erase(v.begin() + maxIndex + 1);
		}
		gain.erase(gain.begin() + maxIndex);
		v.erase(v.begin() + maxIndex);
		if (maxIndex > 0) {
			gain.erase(gain.begin() + maxIndex - 1);
			v.erase(v.begin() + maxIndex - 1);
		}
	}
	cout << result << endl;
	return 0;
}