#ifdef _WIN32
#define DEBUG	1
#endif

#define _CRT_SECURE_NO_WARNINGS
#define MATH_PI 3.14159265358979323846264338327950288419716939
#include <iostream>
#include <string.h>
#include <fstream>
using namespace std;

#ifdef DEBUG
#include "include.h"
#else
inline void wait(){}
inline void dout(const char *arg, ...){}
#endif

//template<typename T>inline void swap(T a, T b){T t = a; a = b; b = t;}

#define CLIP(ptr, min, max) {if((min)<=(max)){if(ptr<(min)){ptr=(min);}if(ptr>(max)){ptr=(max);}}}
#define max(a, b) a > b ? a : b;
#define min(a, b) a > b ? b : a;
#define Sin(deg) sin((deg)*MATH_PI/180)
#define Cos(deg) cos((deg)*MATH_PI/180)
#define Tan(deg) tan((deg)*MATH_PI/180)
#define Rad(deg) ((deg)*MATH_PI/180)
#define ff(param, num) for(int param=0 ; param<num ; ++param)
#define fi(num) for(int i=0 ; i<num ; ++i)
#define fj(num) for(int j=0 ; j<num ; ++j)
#define fk(num) for(int k=0 ; k<num ; ++k)
#define fl(num) for(int l=0 ; l<num ; ++l)
#define fn(num) for(int n=0 ; n<num ; ++n)
#define ffr(param, num) for(int param=num-1 ; param>=0 ; --param)
#define fir(num) for(int i=num-1 ; i>=0 ; --i)
#define fjr(num) for(int j=num-1 ; j>=0 ; --j)
#define fkr(num) for(int k=num-1 ; k>=0 ; --k)
#define flr(num) for(int l=num-1 ; l>=0 ; --l)
#define fnr(num) for(int n=num-1 ; n>=0 ; --n)
#define gi(p) Scanf("%d", p)
#define gi2(p1, p2) Scanf2("%d %d", p1, p2)
#define gi3(p1, p2, p3) Scanf3("%d %d %d", p1, p2, p3)
#define gi4(p1, p2, p3, p4) Scanf4("%d %d %d %d", p1, p2, p3, p4)
#define gll(p) Scanf("%lld", p)
#define gll2(p1, p2) Scanf2("%lld %lld", p1, p2)
#define gll3(p1, p2, p3) Scanf3("%lld %lld %lld", p1, p2, p3)
#define gll4(p1, p2, p3, p4) Scanf4("%lld %lld %lld %lld", p1, p2, p3, p4)
#define gf(p) Scanf("%f", p)
#define gf2(p1, p2) Scanf2("%f %f", p1, p2)
#define gf3(p1, p2, p3) Scanf3("%f %f %f", p1, p2, p3)
#define gf4(p1, p2, p3, p4) Scanf4("%f %f %f %f", p1, p2, p3, p4)
#ifdef DEBUG
#define gc(buf) fscanf(local_in.fp, "%s", buf)
#define Scanf(expr, p) fscanf(local_in.fp, expr, &p)
#define Scanf2(expr, p1, p2) fscanf(local_in.fp, expr, &p1, &p2)
#define Scanf3(expr, p1, p2, p3) fscanf(local_in.fp, expr, &p1, &p2, &p3)
#define Scanf4(expr, p1, p2, p3, p4) fscanf(local_in.fp, expr, &p1, &p2, &p3, &p4)
#else
#define gc(buf) Scanf("%s", buf)
#define Scanf(expr, p) scanf(expr, &p)
#define Scanf2(expr, p1, p2) scanf(expr, &p1, &p2)
#define Scanf3(expr, p1, p2, p3) scanf(expr, &p1, &p2, &p3)
#define Scanf4(expr, p1, p2, p3, p4) scanf(expr, &p1, &p2, &p3, &p4)
#endif
#define ans(p) cout << p << endl;
void CombSort(int N, int *ar, int order_ascending=1){int h=int(N/1.3);int flag;int i;while(true){flag=0;for(i=0;i+h<N;++i){if((order_ascending&&ar[i]>ar[i+h])||(!order_ascending&&ar[i]< ar[i+h])){swap<int>(ar[i],ar[i+h]);flag=1;}}if(h==1&&!flag)break;if(h==9||h==10) h=11;if(h>1)h=int(h/1.3);}}
int EuclideanAlgorithm(int N, int *ar){fn(N-1){while(true){if(ar[n]%ar[n+1]==0||ar[n+1]%ar[n]==0){ar[n+1]=min(ar[n],ar[n+1]);break;}if(ar[n]>ar[n+1]){ar[n]%= ar[n+1];}else{ar[n+1]%=ar[n];}}}return ar[N-1];}

int X[100000], dist[100000];
signed main() {

	int ii, jj, kk, ll;
	int ret = 0;
	int flag = 0;
	char cbuf[8192];

	int N, M;
	gi2(N, M);

	if(N >= M){
		ans(0);
		return 0;
	}

	fi(M){
		gi(X[i]);
	}
	CombSort(M, X);
	fi(M-1){
		dist[i] = X[i+1] - X[i];
	}
	if(M >= 3) CombSort(M-1, dist);

	int sum = 0;
	fi(M-1-(N-1)){
		sum += dist[i];
	}
	ans(sum);

	wait();

	return 0;
}

Measured memory_bytes: 1024