#include <stdio.h>
#include <math.h>
#include <iostream>
#include <string.h>
#include <stdlib.h>
using namespace std;
#define NMax 50000
int main()
{
	long int n, k;
	long long int v[NMax+1];
	FILE *f = fopen("deque.in", "r");
	FILE *g = fopen("deque.out", "w");
	fscanf(f, "%ld %ld", &n, &k);
	for (int i=0; i<n; i++)
		fscanf(f, "%lld", &v[i]);
	long long int s = 0;
	for (int i=0; i<n && n-i >= 3; i++)
	{
		int min = v[i];
		for (int j=i+1; j<i+3; j++)
		{
			if (min > v[j])
				min = v[j];
		}
		s = s + min;
	}
	fprintf(g, "%lld", s);
	fclose(f);
	fclose(g);
	return 0;
}