#include <bits/stdc++.h> 
using namespace std;

typedef long long ll;
#define pa pair<int, int> 
#define pal pair<long long, long long>
#define pali pair<long long, int>
#define pad pair<double, double>
#define pb push_back
#define mp make_pair 
#define COUT(v) for(int64_t i=0;i<(v).size();i++){ cout << v.at(i) << endl;}
#define REP(i, n) for(int64_t i=0;i<n;i++)
#define VIN(v) for(int64_t i=0;i<(v).size();i++){cin >> (v).at(i);}

typedef vector<bool> bvec;
typedef vector<int> ivec;
typedef vector<long long> lvec;
typedef vector<double> dvec;
typedef vector<pa> pavec;
typedef vector<pali> palivec;
typedef vector<pal>palvec;


typedef vector< vector<bool> > bmat;
typedef vector< vector<int> > imat;
typedef vector< vector<long long> > lmat;

typedef vector<string> svec;
typedef vector< vector<string> > smat;
const ll infll = (1LL << 60) - 1;
const int inf = (1 << 30) - 1;
const int MOD = 1000000007;


ll gcd(ll x, ll y){
    ll r = x%y;
    if(r == 0) return y;
    else return gcd(y, r);
} 

ll lcm(ll x, ll y){
    return x*y/gcd(x,y);
}

lvec mfactor(ll n){
    bvec ip(n, true);
    lvec mf(n, -1);
    ip[0]=false; ip[1]=false; mf[0]=0; mf[1]=1;
    REP(i, n){
        if(ip[i]){
            mf[i] = i;
            for(ll j=i*i; j<n; j+=i){
                ip[j] = false;
                if(mf[j] == -1) mf[j]=i;
            }
        }
    }
    return mf;
}

palivec get_prime(ll n, const lvec &mf){
    palivec plist;
    while(n != 1){
        int cnt=0;
        ll m=mf[n];
        while(mf[n] == m){
            cnt++;
            n/=m;
        }
        plist.pb(pali(m, cnt));
    }
    return plist;
}

void COMinit(int m, lvec &fac, lvec &finv){
    lvec inv(m);
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < m; i++){
        fac[i] = fac[i - 1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
        finv[i] = finv[i - 1] * inv[i] % MOD;
    }
}
ll pow_mod(ll a, ll n){
  ll x = 1;
  while(n > 0){
    if(n&1){
      x = x*a % MOD;
    }
    a = a*a %MOD;
    n >>= 1;
  }
  return x;
}
ll COM(int n, int k, const lvec& fac, const lvec &finv){
    if (n < k) return 0;
    if (n < 0 || k < 0) return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}


ll modinv(ll a, ll m) {
    ll b = m, u = 1, v = 0;
    while (b) {
        ll t = a / b;
        a -= t * b; swap(a, b);
        u -= t * v; swap(u, v);
    }
    u %= m;
    if (u < 0) u += m;
    return u;
}

ll P(ll n, ll m){
    ll s = 1;
    ll cnt=n;
    REP(i, n-m){
        s*=cnt % MOD;
        s%=MOD;
        --cnt;
    }
    return s;    
}

ll delta(const lvec &x, ll p, int d){
    ll s=0;
    REP(i, x.size()){
        s+= -2*(x[i]-p)*d + d*d;
    }
    return s;
}

ll delta(const lvec &x, ll p){
    ll s=0;
    REP(i, x.size()) s+= (x[i]-p)*(x[i]-p);
    return s;
}

ll find(const lvec &x, ll p){
    ll mm = 1e5+1;
    ll res = x[0];
    REP(i, x.size()){
        if(abs(p-x[i]) < mm){
            res=x[i];
            mm = abs(p-x[i]);
        }
    }
    return res;
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    ll n, m;
    cin >> n >> m;
    lvec x(m);
    VIN(x);
    if (n >= m){
        cout << 0  << endl;
        return 0;
    }
    sort(x.begin(), x.end());
    lvec diff(m-1), init(n);
    REP(i, m-1) diff[i] = x[i+1]-x[i];
    lvec indices(m-1);
    sort(diff.rbegin(), diff.rend());
    ll s = 0;
    REP(i, n-1) s += diff[i];
    cout << x[m-1] - x[0] - s << endl;
}


Measured memory_bytes: 3200