#include<bits/stdc++.h>
#include<atcoder/all>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
using namespace atcoder;
typedef long long ll;
typedef vector<int> vi;
typedef vector<long long> vl;
typedef vector<vector<int>> vvi;
typedef vector<vector<long long>> vvl;
typedef long double ld;
typedef pair<int, int> P;

ostream& operator<<(ostream& os, const modint& a) {os << a.val(); return os;}
template <int m> ostream& operator<<(ostream& os, const static_modint<m>& a) {os << a.val(); return os;}
template<typename T> istream& operator>>(istream& is, vector<T>& v){int n = v.size(); assert(n > 0); rep(i, n) is >> v[i]; return is;}
template<typename U, typename T> ostream& operator<<(ostream& os, const pair<U, T>& p){os << p.first << ' ' << p.second; return os;}
template<typename T> ostream& operator<<(ostream& os, const vector<T>& v){int n = v.size(); rep(i, n) os << v[i] << (i == n - 1 ? "\n" : " "); return os;}
template <typename T> ostream& operator<<(ostream& os, const vector<vector<T>>& v){int n = v.size(); rep(i, n) os << v[i] << (i == n - 1 ? "\n" : ""); return os;}

int main(){
	int l, n;
	cin >> l >> n;
	vi a(n);
	cin >> a;
	a.push_back(l);
	n++;
	vi dp1(2 * l + 1, 0), dp2(2 * l + 1, 0);
	dp1[l] = 1;
	rep(i, n){
		vi old1(2 * l + 1, 0), old2(2 * l + 1, 0);
		int dist = a[i];
		if(i > 0) dist -= a[i - 1];
		swap(dp1, old1);
		swap(dp2, old2);
		for(int from = 0; from <= 2 * l; from++){
			if(old1[from] or old2[from]){
				int plus = from + dist;
				if(plus >= 0 and plus <= 2 * l){
					dp1[plus] = 1;
				}
				if(i == 0) continue;
				int minus = from - dist;
				if(minus >= 0 and minus <= 2 * l){
					dp2[minus] = 1;
				}
			}
		}
//		cout << dp1 << dp2;
	}
	int ans = l;
	for(int from = 0; from <= 2 * l; from++){
		if(dp2[from]) ans = min(ans, abs(from - l));
	}
	cout << ans << "\n";
	return 0;
}