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

/* FastIO by rsk0315 (last update: 2018/02/21 00:36) */

namespace FastIn {
    static constexpr size_t BUF_SIZE=1<<17, INT_LEN=24, FLT_LEN=400;
    static char buf[BUF_SIZE|1]={}, *pos=buf, *endbuf=nullptr;
    FILE *fin;

    inline bool rebuffer() {
        // returns true <=> there is at least one unread character
        size_t rest=endbuf-pos;
        if (buf+rest > pos) {
            // buf[:pos] and buf[-pos:] are overlapping, which std::memcpy()
            // causes undefined behavior.
            return true;
        }

        std::memcpy(buf, pos, rest);
        pos = buf;
        size_t len=std::fread(pos+rest, 1, BUF_SIZE-rest, fin);
        *(endbuf = buf + (rest+len)) = 0;

        return *pos;
    }

    inline bool scan(char &in) {
        if ((in = *pos)) {
            ++pos;
            return true;
        }

        return rebuffer() && (in = *pos++);
    }

    inline bool scan(char *in) {
        if ((*in = *pos) == 0) {
            if (rebuffer() && (*in = *pos) == 0) {
                return false;
            }
        }
        ++in;
        while (true) {
            if ((*in = *pos++) == 0) {
                if (rebuffer() && (*in = *pos++) == 0) {
                    return true;
                }
            }
            ++in;
        }
    }

    inline bool scan(double &in) {
        if (pos + FLT_LEN >= endbuf && !rebuffer()) {
            in = 0.0;
            return false;
        }

        char *tmp;
        in = std::strtod(pos, &tmp);
        pos = tmp;
        return true;
    }

    template <class Int>
    inline bool scan(Int &in) {
        in = 0;

        // assume that no redundant whitespace appears
        if (pos + INT_LEN >= endbuf && !rebuffer()) {
            return false;
        }

        if (std::is_signed<Int>::value) {
            if (*pos == '-') {
                in = ~*++pos+'1';
                while (*++pos >= '0') {
                    in = in*10 + ~*pos+'1';
                }
                ++pos;
                return true;
            }
        }

        // assume that numbers are separated by the character whose value is
        // less than '0' (e.g. whitespaces, newlines)
        do {
            in = in*10 + *pos-'0';
        } while (*++pos >= '0');
        ++pos;
        return true;
    }

    class Scanner {

    public:
        Scanner(FILE *fin=stdin) {
            FastIn::fin = fin;
            endbuf = pos + std::fread(buf, 1, BUF_SIZE, fin);
        }

        template <class T>
        inline bool scan(T &in) {
            return FastIn::scan(in);
        }

        template <class First, class... Rest>
        inline bool scan(First &in, Rest &...ins) {
            return scan(in) && scan(ins...);
        }
    };
}

template <class RandomIt>
auto rsort(RandomIt first, RandomIt last)
-> typename std::enable_if<std::is_integral<
        typename RandomIt::value_type>::value>::type
{
    using IntType = typename RandomIt::value_type;
    intmax_t cur_shift=0, max_exp=sizeof(IntType);
    std::vector<IntType> work(last-first);
    for (int i=0; i<max_exp; ++i) {
        size_t num[256]={};

        for (RandomIt it=first; it<last; ++it)
            ++num[((*it)>>cur_shift)&255];

        {
            size_t tmp=0;
            for (int j=0; j<256; ++j)
                std::swap(tmp, num[j]+=tmp);
        }

        for (RandomIt it=first; it<last; ++it)
            work[num[((*it)>>cur_shift)&255]++] = *it;

        {
            size_t j=0;
            for (RandomIt it=first; it<last; ++it)
                *it = work[j++];
        }

        cur_shift += 8;
    }

    if (!std::is_signed<IntType>::value) return;

    RandomIt mid=last;
    for (RandomIt it=last; --it>=first;)
        if (*it >= 0) {
            mid = ++it;
            break;
        }

    if (mid == last)
        return;

    std::copy(first, mid, work.begin()+(last-mid));
    std::copy(mid, last, work.begin());
    std::copy(work.begin(), work.end(), first);
}

long N, M;
vector<long> l(100010);

int main(){
    FastIn::Scanner in;

    in.scan(N, M);
    if(M <= N)return 0 & puts("0");

    for(int i = 0; i < M; ++i)in.scan(l[i]);

    rsort(begin(l), begin(l) + M);
    for(auto i = M - 1; i > 0; --i)l[i] -= l[i - 1];
    l[0] = 0;
    nth_element(rend(l) - M, rend(l) - N, rend(l));

    printf("%ld\n", accumulate(begin(l) + N - 1, begin(l) + M, 0L));
}

Measured memory_bytes: 1920