"use strict"
const create = () => {
    const input = require("fs").readFileSync("/dev/stdin", "utf8").trim().split("\n");
    const res = {
        "list": input, "index": 0, "max": input.length,
        "hasNext": function () { return this.index < this.max },
        "next": function () { if (!this.hasNext()) { throw "ArrayIndexOutOfBoundsException"; } else { return this.list[this.index++]; } }
    };
    return res;
}
const o = create();
const next = () => o.next()
const nextInt = () => parseInt(o.next())
const nextStrArray = () => o.next().split(" ")
const nextIntArray = () => o.next().split(" ").map(el => parseInt(el))
const nextCharArray = () => o.next().split("")
const hasNext = () => o.hasNext()
const myout = (...x) => console.log(...x)
const Main = () => {
    const [N, M] = nextIntArray()
    const x = nextIntArray()
    x.sort((a, b) => a - b)
    let diff = []
    for (let i = 1; i < M; i++) {
        diff.push(x[i] - x[i - 1])
    }
    diff.sort((a, b) => a - b)
    let ans = 0
    for (let i = 0; i < M - N; i++) {
        ans += diff[i]
    }
    console.log(ans)

}
Main()
