import java.util.ArrayList;
import java.util.Arrays;
import java.util.Scanner;

public class Kaitenzushi { 
	static int D = 0;

	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		int N = s.nextInt();
		ArrayList<Integer> V = new ArrayList<>();
		for(int i = 0;i < N;i++){
			V.add(s.nextInt());
			
		}
		ArrayList<Integer> X = new ArrayList<>(V);
		s.close();
		Object[] W = V.toArray();
		
		for(;;){
			W = X.toArray();
			Arrays.sort(W);
			int max = (int)W[W.length-1];
			int d = V.indexOf(max);
			W = V.toArray();
			D += max;
			//System.out.println(max + "\t" + N + "\t" + d);
			if(max == 0 || N == 1){
				break;
			}else if(d > N-3){
				for(int i = N-1;i >= d-1;i--){
					X.set(i, 0);
				}
			}else if(d < 2){
				for(int i = 0;i <= d+1;i++){
					X.set(i, 0);
				}
			}else{
				X.set(d+1, 0);
				X.set(d, 0);
				X.set(d-1,0);
			}
		}
		
		System.out.println(D);
	}
}