1906

#include <iostream>
#include <fstream>
#include <vector>
#include <algorithm>

using namespace std;

int main() {
    ifstream fin("input.txt");
    ofstream fout("output.txt");

    int len, n;
    fin >> len >> n;
    vector<int> vec(n + 2);
    vec[0] = 0; for (int i = 1; i <= n; i++) fin >> vec[i]; vec[++n] = len;
    vector<vector<int>> dp(n + 1, vector<int>(n+1, 1e9));
    for (int i = 0; i <= n; i++) dp[i][i] = dp[i][i+1 <= n ? i + 1 : i] = 0;
    for (int l = 2; l <= n; l++) {
        for (int beg = 0; beg + l <= n; beg++) {
            for (int k = 1; k < l; k++) {
                dp[beg][beg + l] = min(dp[beg][beg + l], dp[beg][beg + k] + dp[beg + k][beg + l] + vec[beg + l] - vec[beg]);
            }
        }
    }
    fout << dp[0][n] << endl;

    fin.close();
    fout.close();
    return 0;
}