"use strict";
function Main(input) {
    let cin = new Inputer(input);
    let N = cin.getInt(), M = cin.getInt();
    let X = cin.getIntArray(M);
    let P = new Array(M - 1);
    X.sort((a, b) => a - b);
    let ans;
    if (N == 1) ans = X[M - 1] - X[0];
    else if (N == M) ans = 0;
    else {
        for (let n = 0; n < M - 1; n++) P[n] = X[n + 1] - X[n];
        P.sort((a, b) => a - b);
        ans = P.reduce((ac, v, i) => ac + (i < M - N ? v : 0), 0);
    }
    console.log(ans);
}
class Inputer{constructor(t){if(typeof t=='undefined')throw new Error('引数（stdin）を省略できません！');this.cnt=0,this.items=t.split(/\n|\s/)}getInt(){return this.items[this.cnt++]-0}getIntArray(t){if(typeof t=='undefined')throw new Error('引数（1以上）を省略できません！');let r=new Array(t);for(let e=0;e<t;e++)r[e]=this.getInt();return r}getStr(){return this.items[this.cnt++]}getStrArray(t){if(typeof t=='undefined')throw new Error('引数（1以上）を省略できません！');let r=new Array(t);for(let e=0;e<t;e++)r[e]=this.getStr();return r}}
if (typeof window == "undefined") Main(require("fs").readFileSync("/dev/stdin", "utf8"));