#include <bits/stdc++.h>
using namespace std;
void readi(int &x) {
  int v = 0, f = 1;
  char c = getchar();
  while (!isdigit(c) && c != '-') c = getchar();
  if (c == '-')
    f = -1;
  else
    v = v * 10 + c - '0';
  while (isdigit(c = getchar())) v = v * 10 + c - '0';
  x = v * f;
}
void readll(long long &x) {
  long long v = 0ll, f = 1ll;
  char c = getchar();
  while (!isdigit(c) && c != '-') c = getchar();
  if (c == '-')
    f = -1;
  else
    v = v * 10 + c - '0';
  while (isdigit(c = getchar())) v = v * 10 + c - '0';
  x = v * f;
}
void readc(char &x) {
  char c;
  while ((c = getchar()) == ' ')
    ;
  x = c;
}
void writes(string s) { puts(s.c_str()); }
void writeln() { writes(""); }
void writei(int x) {
  if (!x) putchar('0');
  char a[25];
  int top = 0;
  while (x) {
    a[++top] = (x % 10) + '0';
    x /= 10;
  }
  while (top) {
    putchar(a[top]);
    top--;
  }
}
void writell(long long x) {
  if (!x) putchar('0');
  char a[25];
  int top = 0;
  while (x) {
    a[++top] = (x % 10) + '0';
    x /= 10;
  }
  while (top) {
    putchar(a[top]);
    top--;
  }
}
inline long long inc(int &x) { return ++x; }
inline long long inc(long long &x) { return ++x; }
inline long long inc(int &x, long long y) { return x += y; }
inline long long inc(long long &x, long long y) { return x += y; }
inline double inc(double &x, double y) { return x += y; }
inline long long dec(int &x) { return --x; }
inline long long dec(long long &x) { return --x; }
inline long long dec(int &x, long long y) { return x -= y; }
inline long long dec(long long &x, long long y) { return x -= y; }
inline double dec(double &x, double y) { return x -= y; }
inline long long mul(int &x) { return x = ((long long)x) * x; }
inline long long mul(long long &x) { return x = x * x; }
inline long long mul(int &x, long long y) { return x *= y; }
inline long long mul(long long &x, long long y) { return x *= y; }
inline double mul(double &x, double y) { return x *= y; }
inline long long divi(int &x) { return x = sqrt(x); }
inline long long divi(long long &x) { return x = sqrt(x); }
inline long long divi(int &x, long long y) { return x /= y; }
inline long long divi(long long &x, long long y) { return x /= y; }
inline double divi(double &x, double y) { return x /= y; }
inline long long mod(int &x, long long y) { return x %= y; }
inline long long mod(long long &x, long long y) { return x %= y; }
long long n, m, i, j, t[500005], a[500005], l[500005], r[500005], ans,
    vis[500005];
priority_queue<pair<long long, long long> > pq;
int main() {
  ios_base::sync_with_stdio(false);
  ;
  cin >> m >> n;
  if ((1) <= ((n)))
    for (((i)) = (1); ((i)) <= ((n)); ((i))++) {
      cin >> t[i];
    }
  stable_sort(t + 1, t + n + 1);
  if ((1) <= ((n - 1)))
    for (((i)) = (1); ((i)) <= ((n - 1)); ((i))++) {
      a[i] = t[i + 1] - t[i];
      pq.push(make_pair(-a[i], i));
      l[i] = i - 1;
      r[i] = i + 1;
    }
  a[0] = a[n] = 0x1bbbbbbbbbbbbbbbll;
  r[0] = 1;
  l[n] = n - 1;
  while (m) {
    long long x = -pq.top().first, y = pq.top().second;
    pq.pop();
    if (vis[y]) continue;
    m--;
    ans += x;
    pq.push(make_pair(-(a[y] = a[l[y]] + a[r[y]] - x), y));
    vis[l[y]] = vis[r[y]] = 1;
    r[l[l[y]]] = y;
    l[y] = l[l[y]];
    l[r[r[y]]] = y;
    r[y] = r[r[y]];
  }
  cout << ans;
  return 0;
}
