#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; }