//============================================================================
// Name        : 1090.cpp
// Author      : 
// Version     :
// Copyright   : Your copyright notice
// Description : Hello World in C++, Ansi-style
//============================================================================

#include <stdio.h>
#include <vector>
using namespace std;

#define X 1000000000
#define N 1001
int data[N];
int n;
int des[N];
int sum;
int flag[N];

class BigInteger {
private:
	vector<int> data;
public:
	BigInteger(int number) {
		data.push_back(number);
	}

	void doubleSelf() {
		int last = 0;
		for (int i = 0; i < data.size(); i++) {
			int sum = (data[i] << 1) + last;
			if (sum >= X) {
				data[i] = sum - X;
				last = 1;
			} else {
				data[i] = sum;
				last = 0;
			}
		}

		if (last > 0) {
			data.push_back(last);
		}
	}

	void add(const BigInteger& other) {
		int last = 0;
		for (int i = 0; i < data.size(); i++) {
			if (i < other.data.size()) {
				int sum = data[i] + other.data[i] + last;
				if (sum >= X) {
					data[i] = sum - X;
					last = 1;
				} else {
					data[i] = sum;
					last = 0;
				}
			} else {
				int sum = data[i] + last;
				if (sum >= X) {
					data[i] = sum - X;
					last = 1;
				} else {
					data[i] = sum;
					last = 0;
				}
			}
		}
		for (int i = data.size(); i < other.data.size(); i++) {
			int sum = other.data[i] + last;
			if (sum >= X) {
				data.push_back(sum - X);
				last = 1;
			} else {
				data.push_back(sum);
				last = 0;
			}
		}
		if (last > 0) {
			data.push_back(last);
		}
	}

	void output() {
		printf("%d", data[data.size() - 1]);
		for (int i = data.size() - 2; i >= 0; i--) {
			printf("%9.9d", data[i]);
		}
		puts("");
	}
};

void calStep(int n) {
	if (n < 0) {
		return;
	}
	if (data[n] != des[n]) {
		des[n - 1] = 1;
		flag[n] = 1;
	}
	calStep(n - 1);
}

int main() {
	scanf("%d", &n);
	int lastOne = 0;
	for (int i = 0; i < n; i++) {
		scanf("%d", &data[i]);
		if (data[i]) {
			lastOne = i;
		}
	}
	sum = 0;
	calStep(lastOne);
	BigInteger out(0);
	BigInteger temp(1);
	for (int i = 0; i <= lastOne; i++) {
		if (flag[i]) {
			out.add(temp);
		}
		temp.doubleSelf();
	}
	out.output();
	return 0;
}