#include <stdio.h>
int minim(int a,int b,int c)
{
	if (a<=b && a<=c)
		return a;
	if (b<=a && b<=c)
		return b;
	return c;
}
int main()
{
	freopen("deque.in","r",stdin);
	freopen("deque.out","w",stdout);
	int n,k,a[500005],i,s=0;
	scanf("%d%d",&n,&k);
	for (i=1; i<=n; i++)
		scanf("%d",&a[i]);
	for (i=1; i<=n-2; i++)
		s+=minim(a[i],a[i+1],a[i+2]);
	printf("%d",s);
	return 0;
}