#include<algorithm>
#include<iostream>
#include<vector>
#include<map>
#include<cassert>
#include<set>
#include<queue>
#include<cstdint>
#include<unordered_map>
using namespace std;
using ll = long long;
#include<stack>

#include<bitset>
const int mx = 2e5;
using B = bitset<mx>;
int main(){
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    ll l,n;
    cin>>l>>n;
    vector<ll> x(n);
    for(int i = 0;i<n;i++) cin>>x[i];
    vector<B> dp(4);
    dp[0].set(0);
    ll sum = 0;
    vector<ll> d;
    d.push_back(x[0]);
    for(int i = 1;i<n;i++) d.push_back(x[i]-x[i-1]);
    d.push_back(l-x.back());
    for(int i = 0;i<d.size();i++){
        vector<B> ndp(4);
        ndp[0] = (dp[3]);
        ndp[1] = (dp[1]<<d[i]) | (dp[0]<<d[i]) | (dp[3]<<d[i]);
        ndp[3] = dp[1] | dp[3];
        swap(ndp,dp);
    }
    ll ans = 1e18;
    sum = l;
    for(int i = 0;i<mx;i++) if(dp[3][i]){
       // cout<<i<<endl;
        ans = min(ans,abs(sum-2*i));
    }
    cout<<ans<<endl;
}