import java.util.Scanner;

public class Main {

	@SuppressWarnings("resource")
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
		int n = scanner.nextInt();
		int v[]=new int[n+2];
		for(int i=2;i<=n+1;i++){
			v[i]=scanner.nextInt();
		}
		dfs(0,0,v);
		System.out.println(sum);
	}

	static int sum=0;
	private static void dfs(int i,int sc,int[] v) {
		sum=Math.max(sum, sc);
		for(int j=i+2;j<v.length;j++){
			dfs(j,sc+v[j],v);
		}
		
	}
}