//varianta divide si impera (gandita repede, posibile bug-uri nerezolvate)
#include <cstdio>
#include <vector>
#define MX 5000001
#define minim(a,b) ((a<b) ? a : b)
using namespace std;

FILE *fin, *fout;
int n,k;
long long suma;
vector<int> v;
int a,b,c;

inline int divimp(int left, int right)
{
    if(left == right) return v[left];
    int mid = (left+right)>>1;

    return minim(divimp(left, mid), divimp(mid+1, right));
}

int main()
{
    fin = fopen("deque.in", "r");
    fout = fopen("deque.out", "w");

    int i,x;
    v.push_back(0);
    fscanf(fin, "%d%d", &n, &k);

    fscanf(fin, "%d", &a);
    for(i=1; i<=k-2; i++)
    {
        fscanf(fin, "%d", &x);
        v.push_back(x);
    }
    fscanf(fin, "%d", &c);
    b = divimp(1, v.size()-1);
    //fprintf(fout, "%d", minim(minim(a,b), c));

    suma += minim(minim(a,b), c);
    a=b; b=c;
    for(i=4; i<=n; i++)
    {
        fscanf(fin, "%d", &c);
        suma += minim(minim(a,b), c);
        a=b; b=c;
    }
    fprintf(fout, "%lld", suma);


    fclose(fin); fclose(fout);
    return 0;
}