#include<bits/stdc++.h>
using namespace std;
void *wmem;
void sortF_L(int N, unsigned A[], void *mem = wmem){
  int bt, i, m, *sz;
  unsigned *arr, c;
  if(N < 256){
    sort(A, A+N);
    return;
  }
  bt = sizeof(unsigned) * 8;
  arr = (unsigned*)mem;
  sz = (int*)(arr+N);
  for(m=0;m<bt;m+=8){
    for(i=0;i<257;i++){
      sz[i] = 0;
    }
    for(i=0;i<N;i++){
      sz[ 1+((A[i]>>m)&255u) ]++;
    }
    for(i=1;i<257;i++){
      sz[i] += sz[i-1];
    }
    for(i=0;i<N;i++){
      c = ((A[i]>>m)&255u);
      arr[sz[c]++] = A[i];
    }
    swap(A, arr);
  }
}
void sortF_L(int N, int A[], void *mem = wmem){
  int *arr, i, x, y, z;
  unsigned *send;
  if(N < 256){
    sort(A, A+N);
    return;
  }
  send = (unsigned*)A;
  sortF_L(N, send, mem);
  if(A[0] < 0 || A[N-1] >= 0){
    return;
  }
  x = 0;
  y = N;
  while(x < y){
    z = (x+y) / 2;
    if(A[z] < 0){
      y = z;
    }
    else{
      x = z+1;
    }
  }
  arr = (int*)mem;
  z = 0;
  for(i=x;i<N;i++){
    arr[z++] = A[i];
  }
  for(i=0;i<x;i++){
    arr[z++] = A[i];
  }
  for(i=0;i<N;i++){
    A[i] = arr[i];
  }
}
void sortF_L(int N, unsigned long long A[], void *mem = wmem){
  int bt, i, m, *sz;
  unsigned c;
  unsigned long long *arr;
  if(N < 512){
    sort(A, A+N);
    return;
  }
  bt = sizeof(unsigned long long) * 8;
  arr = (unsigned long long*)mem;
  sz = (int*)(arr+N);
  for(m=0;m<bt;m+=8){
    for(i=0;i<257;i++){
      sz[i] = 0;
    }
    for(i=0;i<N;i++){
      sz[ 1+((A[i]>>m)&255u) ]++;
    }
    for(i=1;i<257;i++){
      sz[i] += sz[i-1];
    }
    for(i=0;i<N;i++){
      c = ((A[i]>>m)&255u);
      arr[sz[c]++] = A[i];
    }
    swap(A, arr);
  }
}
void sortF_L(int N, long long A[], void *mem = wmem){
  int i, x, y, z;
  long long *arr;
  unsigned long long *send;
  if(N < 512){
    sort(A, A+N);
    return;
  }
  send = (unsigned long long*)A;
  sortF_L(N, send, mem);
  if(A[0] < 0 || A[N-1] >= 0){
    return;
  }
  x = 0;
  y = N;
  while(x < y){
    z = (x+y) / 2;
    if(A[z] < 0){
      y = z;
    }
    else{
      x = z+1;
    }
  }
  arr = (long long*)mem;
  z = 0;
  for(i=x;i<N;i++){
    arr[z++] = A[i];
  }
  for(i=0;i<x;i++){
    arr[z++] = A[i];
  }
  for(i=0;i<N;i++){
    A[i] = arr[i];
  }
}
inline void rd(int &x){
  int k, m=0;
  x=0;
  for(;;){
    k = getchar_unlocked();
    if(k=='-'){
      m=1;
      break;
    }
    if('0'<=k&&k<='9'){
      x=k-'0';
      break;
    }
  }
  for(;;){
    k = getchar_unlocked();
    if(k<'0'||k>'9'){
      break;
    }
    x=x*10+k-'0';
  }
  if(m){
    x=-x;
  }
}
inline void wt_L(char a){
  putchar_unlocked(a);
}
inline void wt_L(int x){
  char f[10];
  int m=0, s=0;
  if(x<0){
    m=1;
    x=-x;
  }
  while(x){
    f[s++]=x%10;
    x/=10;
  }
  if(!s){
    f[s++]=0;
  }
  if(m){
    putchar_unlocked('-');
  }
  while(s--){
    putchar_unlocked(f[s]+'0');
  }
}
char memarr[96000000];
int N;
int M;
int X[100000];
int main(){
  int KL2GvlyY, i, j, res;
  wmem = memarr;
  rd(N);
  rd(M);
  {
    int Lj4PdHRW;
    for(Lj4PdHRW=0;Lj4PdHRW<M;Lj4PdHRW++){
      rd(X[Lj4PdHRW]);
    }
  }
  if(N > M){
    N = M;
  }
  N--;
  sortF_L(M, X);
  res = X[M-1] - X[0];
  M--;
  for(i=0;i<M;i++){
    X[i] = X[i+1] - X[i];
  }
  sortF_L(M, X);
  for(KL2GvlyY=0;KL2GvlyY<N;KL2GvlyY++){
    res -= X[--M];
  }
  wt_L(res);
  wt_L('\n');
  return 0;
}
// cLay varsion 20190715-1

// --- original code ---
// int N, M, X[1d5];
// {
//   int i, j, res;
//   rd(N,M,X(M));
// 
//   if(N > M) N = M;
//   N--;
// 
//   sortF(M, X);
//   res = X[M-1] - X[0];
// 
//   M--;
//   rep(i,M) X[i] = X[i+1] - X[i];
//   sortF(M, X);
// 
//   rep(N) res -= X[--M];
//   wt(res);
// }


Measured memory_bytes: 1024