#include <bits/stdc++.h>
using namespace std;
#define rep(index,num) for(int index=0;index<num;index++)
#define rep1(index,num) for(int index=1;index<=num;index++)
#define brep(index,num) for(int index=num-1;index>=0;index--)
#define brep1(index,num) for(int index=num;index>0;index--)
#define prin(a) cout<<a<<"\n"
#define kaigyo cout<<"\n"
#define mp(a1,a2) make_pair(a1,a2)
#define ALL(a) (a).begin(),(a).end()
#define rALL(a) (a).rbegin(),(a).rend()
#define SORT(V) sort(ALL(V))
#define REV(V) reverse(ALL(V))
#define RSORT(V) SORT(V);REV(V)

typedef long long ll;
typedef long double ld;
typedef pair<ll,ll> pll;
typedef pair<int,int> pint;
typedef vector<int> vint;
typedef vector<ll> vll;
typedef vector<string> vst;
typedef vector<char> vc;
typedef vector<long double> vd;
typedef vector<pint> vpint;
typedef vector<pll> vpll;
typedef vector<vector<int>> vint2;
ll mod=1e9+7;

const double pi=acos(-1.0);
const ll inf=1LL<<60;
// modint: mod 計算を int を扱うように扱える構造体
template<int MOD> struct Fp {
    long long val;
    constexpr Fp(long long v = 0) noexcept : val(v % MOD) {
        if (val < 0) val += MOD;
    }
    constexpr int getmod() { return MOD; }
    constexpr Fp operator - () const noexcept {
        return val ? MOD - val : 0;
    }
    constexpr Fp operator + (const Fp& r) const noexcept { return Fp(*this) += r; }
    constexpr Fp operator - (const Fp& r) const noexcept { return Fp(*this) -= r; }
    constexpr Fp operator * (const Fp& r) const noexcept { return Fp(*this) *= r; }
    constexpr Fp operator / (const Fp& r) const noexcept { return Fp(*this) /= r; }
    constexpr Fp& operator += (const Fp& r) noexcept {
        val += r.val;
        if (val >= MOD) val -= MOD;
        return *this;
    }
    constexpr Fp& operator -= (const Fp& r) noexcept {
        val -= r.val;
        if (val < 0) val += MOD;
        return *this;
    }
    constexpr Fp& operator *= (const Fp& r) noexcept {
        val = val * r.val % MOD;
        return *this;
    }
    constexpr Fp& operator /= (const Fp& r) noexcept {
        long long a = r.val, b = MOD, u = 1, v = 0;
        while (b) {
            long long t = a / b;
            a -= t * b; swap(a, b);
            u -= t * v; swap(u, v);
        }
        val = val * u % MOD;
        if (val < 0) val += MOD;
        return *this;
    }
    constexpr bool operator == (const Fp& r) const noexcept {
        return this->val == r.val;
    }
    constexpr bool operator != (const Fp& r) const noexcept {
        return this->val != r.val;
    }
    friend constexpr ostream& operator << (ostream &os, const Fp<MOD>& x) noexcept {
        return os << x.val;
    }
    friend constexpr Fp<MOD> modpow(const Fp<MOD> &a, long long n) noexcept {
        if (n == 0) return 1;
        auto t = modpow(a, n / 2);
        t = t * t;
        if (n & 1) t = t * a;
        return t;
    }
};
const int MOD = 1000000007;
using mint = Fp<MOD>;

bool isPrime(ll n){if(n<2)return false;for(ll x = 2;
x*x <=n;x++){if(n%x == 0)return false;}return true;}
ll kaizyo(ll k){ll sum=1;rep1(i,k){sum*=i;sum%=mod;}return sum;}
ll ncr(ll n,ll r){return kaizyo(n)/(kaizyo(r)*kaizyo(n-r));}




int main(){
ios_base::sync_with_stdio(false);cin.tie(NULL);
cout.precision(20);

int n,m;cin>>n>>m;
vint x(m+1);
rep(i,m)cin>>x[i];

x[m]=mod;sort(ALL(x));
vint a(m);
rep(i,m)a[i]=x[i+1]-x[i];
sort(ALL(a));
ll cnt=0;

rep(i,m-n)cnt+=a[i];
prin(cnt);
}


Measured memory_bytes: 3984