#include<iostream>
#include<vector>
#include<algorithm>
#include<cstring>
#include<cassert>
#include<cmath>
#include<ctime>
#include<iomanip>
#include<numeric>
#include<stack>
#include<queue>
#include<map>
#include<unordered_map>
#include<set>
#include<unordered_set>
#include<bitset>
#include<random>
#include<functional>
#include<utility>
using namespace std;
int L,N,X[100];
const int M = (int)1e5;
int dp[2][2*M+1];
void solve()
{
	cin >> L >> N;
	for(int i = 0;i < N;i++) cin >> X[i];

	int cur = 0;
	dp[cur][M] = 1;
	for(int i = 1;i < N;i++)
	{
		int d = X[i]-X[i-1];
		int nxt = 1-cur;
		for(int j = -M;j <= M;j++) dp[nxt][j+M] = 0;
		for(int j = -M;j <= M;j++)
		{
			{
				int nj = j + d;
				if(-M <= nj && nj <= M) dp[nxt][nj+M] |= dp[cur][j+M];
			}
			{
				int nj = j - d;
				if(-M <= nj && nj <= M) dp[nxt][nj+M] |= dp[cur][j+M];
			}
		}
		cur = nxt;
	}
	int ans = (int)2e9;
	for(int j = -M;j <= M;j++) if(dp[cur][j+M])
	{
		int val = j + X[0] - (L-X[N-1]);
		ans = min(ans,abs(val));
	}
	cout << ans << endl;
}
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int tt = 1;
	/* cin >> tt; */
	while(tt--) solve();
}