PROGRAM ATCODER

implicit none
integer :: n, m, i, j, a, ans
integer,allocatable :: x(:), dx(:),dx1(:),bar(:)

read(*,*) n, m
allocate(x(m))
allocate(dx(m-1))
allocate(dx1(m-1))
allocate(bar(n-1))
read(*,*) (x(i), i=1,m)

if(n >= m) then
	ans = 0
	write(*,'(i0)') ans
    stop
end if

call hs(m,x) !昇順に並び替え

do i =1, m-1
	dx(i) = x(i+1) - x(i)
end do

call hs(m-1,dx)

ans = sum(dx)

if (n > 1) then
	ans = ans - sum(dx(m-n+1:m-1))
end if

write(*,'(i0)') ans

END PROGRAM ATCODER


subroutine hs(n,array)
  implicit none
  integer,intent(in) :: n
  integer,intent(inout) :: array(1:n)
  
  integer ::i,k,j,l
  integer :: t
  
  if(n <= 0)then
     write(6,*)"Error, at heapsort"; stop
  endif
  if(n == 1)return
 
  l=n/2+1
  k=n
  do while(k /= 1)
     if(l > 1)then
        l=l-1
        t=array(L)
     else
        t=array(k)
        array(k)=array(1)
        k=k-1
        if(k == 1) then
           array(1)=t
           exit
        endif
     endif
     i=l
     j=l+l
     do while(j <= k)
        if(j < k)then
           if(array(j) < array(j+1))j=j+1
        endif
        if (t < array(j))then
           array(i)=array(j)
           i=j
           j=j+j
        else
           j=k+1
        endif
     enddo
     array(i)=t
  enddo
 
  return
end subroutine hs