#include <iostream>
#include <vector>
using namespace std;
int main(){
	int len, n;
	cin >> len >> n;
	vector<int> a(n + 2);
	vector<vector<int>> dp(n + 2, vector<int>(n + 2, 0));
	//设置dp[i][j]为切割木棍[i, j]段 0|---1|-----2|----3|--4| 最少所需的费用 a[0] = 0 a[1] = x,,, a[n + 1] = len
	a[0] = 0; a[n + 1] = len;
	for(int i = 1; i <= n; i++){
		cin >> a[i];
	}
	//a[i]是分点
	/*这个不对 
	for(int i = 0; i <= n + 1; i++){
		for(int j = i; j <= n + 1 ;j++){
			dp[i][j] = 114514;
			for(int k = i + 1; k < j; k++){
				dp[i][j] = min(dp[i][j], dp[i][k] + dp[k][j] + a[j] - a[i]);
			}
			if(dp[i][j] == 114514) dp[i][j] = 0;
		}
	} */
	// 区间 DP，按长度递增
    for(int l = 2; l <= n + 1; l++){ // 区间长度至少2
        for(int i = 0; i + l <= n + 1; i++){
            int j = i + l;
            dp[i][j] = INT_MAX;
            for(int k = i + 1; k < j; k++){
                dp[i][j] = min(dp[i][j], dp[i][k] + dp[k][j] + a[j] - a[i]);
            }
            if(dp[i][j] == INT_MAX) dp[i][j] = 0;
        }
    }
	cout << dp[0][n + 1] << endl;
	return 0;
}