E

#include <bits/stdc++.h>

using namespace std;

#define int long long

const long long INF = 1e18 + 7;
const int MAXN = 1e2 + 100;
const int N = 1e5 + 10;

int beam, n;
vector<int> a;
array<array<int, MAXN>, MAXN> dp;

int rec(int l, int r) {
    if (dp[l][r] != INF) {
        return dp[l][r];
    }
    int ans = INF;
    for (int m = l + 1; m < r; ++m) {
        ans = min(ans, rec(l, m) + rec(m, r) + a[r] - a[l]);
    }
    return dp[l][r] = ans;
}

signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    cin >> beam >> n;
    a.resize(n + 2);
    a[0] = 0;
    for (int i = 1; i <= n; ++i) {
        cin >> a[i];
    }
    a[n + 1] = beam;
    n += 2;
    for (int i = 0; i < n; ++i) {
        for (int j = 0; j < n; ++j) {
            dp[i][j] = INF;
        }
    }
    for (int i = 0; i < n - 1; ++i) {
        dp[i][i + 1] = 0;
    }
    cout << rec(0, n - 1) << '\n';
}