program prob4
    implicit none
    integer::N,M,i,j,ans
    integer,allocatable::X(:)
    read(*,*) N,M
    allocate(X(M))
    read(*,*) X
    ans = 0
    if(N < M) then
        call heapsort(M,X)
        do i = 1, M-1
            X(i) = X(i+1) - X(i)
        end do
        X(M) = 0
        call heapsort(M,X)
        do i = 1, M - N + 1
            ans = ans + X(i)
        end do
    end if

    write(*,*) ans
    deallocate(X)
    stop
    contains
    subroutine heapsort(n,array)
        implicit none
        integer,intent(in) :: n
        integer,intent(inout) :: array(1:n)
       
        integer ::i,k,j,l
        double precision :: t
       
        if(n.le.0)then
           write(6,*)"Error, at heapsort"; stop
        endif
        if(n.eq.1)return
      
        l=n/2+1
        k=n
        do while(k.ne.1)
           if(l.gt.1)then
              l=l-1
              t=array(L)
           else
              t=array(k)
              array(k)=array(1)
              k=k-1
              if(k.eq.1) then
                 array(1)=t
                 exit
              endif
           endif
           i=l
           j=l+l
           do while(j.le.k)
              if(j.lt.k)then
                 if(array(j).lt.array(j+1))j=j+1
              endif
              if (t.lt.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 heapsort
end program