#include <bits/stdc++.h>
//#include <atcoder/modint>
using namespace std;
//using namespace atcoder;
//using mint = modint998244353;
#define rep(i,n) for(long long i=0;i<n;i++)
#define rep1(i,n) for(long long i=1;i<=n;i++)
#define Rep(i,a,b) for(long long i=a;i<=b;i++)
#define all(a) (a).begin(), (a).end()
#define fst first
#define snd second
typedef unsigned long long ull;
typedef long long ll;
typedef vector<int> vec;
typedef vector<vector<int>> vvec;
typedef vector<long long> vecll;
typedef vector<vector<long long>> vvecll;

//#include<boost/multiprecision/cpp_int.hpp>
//using namespace boost::multiprecision;

const ll mod=998244353;
//const ll mod=1e9+7;
const ll INF=5e18;
const int INT_INF=1e9;
const double pi=3.14159265358979;

//xのn乗をpで割ったあまりを求める
ll modpow(ll x,ll n,ll p=mod){
  if(n==0) return 1;
  if(n%2==1) return (x*modpow(x,n-1,p))%p;
  else{
    ll t=modpow(x,n/2,p);
    assert(t>0);
    return (t*t)%p;
  }
}

//xの逆元(p:素数)
ll inv(ll x,ll p=mod){
  return modpow(x,p-2,p);
}

template <typename T>
int index(vector<T> &a, T x){
	return lower_bound(all(a),x)-a.begin();
}

int idx(int n){
	return n+100000;
}

int main(){
	int l,n;
	cin >> l >> n;
	vec x(n+1,l);
	rep(i,n) cin >> x[i];
	vector<bool> dp(200001,false);
	dp[idx(x[0])]=true;
	rep1(i,n-1){
		vector<bool> ndp(200001,false);
		rep(j,200001){
			if(dp[j]){
				ndp[j+x[i]-x[i-1]]=true;
				ndp[j+x[i-1]-x[i]]=true;
			}
		}
		dp=move(ndp);
	}

	vector<bool> ndp(200001,false);
	rep(j,200001){
		if(dp[j]){
			ndp[j+x[n-1]-x[n]]=true;
		}
	}
	dp=move(ndp);

	int ans;
	rep(i,100001){
		if(dp[idx(i)]||dp[idx(-i)]){
			ans=i;
			break;
		}
	}
	cout << ans << endl;
}