import java.util.Scanner;

public class Nikola4 {
	public static void main(String[] args) {
		Scanner in = new Scanner(System.in);
		String s = in.nextLine();
		String split[] = s.split(" ");
		int [] niza = new int[split.length];

		for(int i = 0; i < split.length; i++)
		{
			niza[i] = Integer.parseInt(split[i]);
		}
		System.out.println(reverse(niza));

	}
	/*public static int reverse(int [] niza)
	{
		int diff = dif(niza);
		for(int i=0; i<niza.length; i++)
		{
			if(Math.abs(niza[i]-niza[i+1])!=diff)
			{

			}
		}
	}*/
	public static int reverse(int[] split)
	{
		int lower = 0;
		int upper = 0;
		int sum = 0;
		int diff;
		boolean temp = false;
		boolean temp2 = false;
		for(int i = 0; i < split.length; i++)
		{
			sum+=split[i];
			if(i<=split.length/2)
				lower+=split[i];
			else
				upper+=split[i];
		}
		/*for(int i=0; i<split.length-1; i++)
		{
			if(split[i] != split[i+1])
				{
					temp = true;
					break;
				}
		}*/
		diff = Math.abs(split[1]-split[2]);
		for(int i=0; i<split.length-1; i++)
		{
			if(split[i]+diff!=split[i+1])
				{
					temp2 = true;
					break;
				}
		}
		if(!temp2)
		{
			return sum;
		}
		else if(lower < upper)
		{
			sum = 0;
			int dif = 0;
			for(int i=0; i<split.length-1; i++)
			{
				//int diff=0;
				while(split[i]>=split[i+1])
				{
					split[i+1]*=2;
				}
				//sum+=split[i];
				//break;
			}
			dif = Math.abs(split[1]-split[2]);
			for(int i=0; i<split.length-1; i++)
			{
				if(split[i]+dif==split[i+1])
					{
						sum+=split[i];

					}
				else
				{
					//sum+=split[i];
					sum+=split[i]*2;
					//System.out.println(i);
				}
			}
			sum+=split[split.length-1];
			return sum;
		}
		else
		{
			sum = 0;
			int dif = 0;
			for(int i=0; i<split.length-1; i++)
			{
				while(split[i]<=split[i+1])
				{
					split[i]*=2;
				}

			}
			dif = Math.abs(split[1]-split[2]);
			//System.out.println(dif);
			for(int i=0; i<split.length-1; i++)
			{
				if(split[i]-dif==split[i+1])
					{
						sum+=split[i];

					}
				else
				{
					sum+=split[i];
					sum+=split[i+1]*2;
					System.out.println(i);
				}
			}
			return sum;
			}
	}
}