#include <stdio.h>
#include <stdlib.h>
int main(){
  int n, m, *d;
  int i,j,buff;
  int min, d_num, d_inside=0;
  int result = 0;
  
  scanf("%d %d",&n,&m);
  
  d=(int*)malloc(sizeof(int)*n);
  for(i=0;i<m;i++){
    scanf("%d",&d[i]);
  }

  //-----sort--------
  for(i=0;i<m-1;i++){
    min = 10000;
    for(j=i;j<m;j++){
      buff=d[j]-d_inside;
      (buff>0)?(buff=buff):(buff=-buff);
      if(buff<min){
	min = buff;
	d_num = j;
      } 
    }
     d_inside=d[d_num];
     buff=d[i];
     d[i]=d[d_num];
     d[d_num]=buff;
  }
  //-----------------
  result += d[0];
  for(i=1;i<n;i++){
    buff=d[i]-d[i-1];
    (buff>0)?(buff=buff):(buff=-buff);
    result += buff;
  }
  
  free(d);
  return 0;
}