#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 4000000000000000001

int main(){
	
	int L,N;
	cin>>L>>N;
	vector<long long> x(N);
	rep(i,N)cin>>x[i];
	x.push_back(L);
	N++;
	for(int i=N-1;i>=1;i--)x[i] -= x[i-1];
	int A = x[0]-x.back();
	x.erase(x.begin());
	x.pop_back();
	N -= 2;
	vector dp(2,vector<bool>(L*2+5,false));
	int mid = L+1;
	dp[0][mid] = true;
	rep(i,N){
		vector ndp(2,vector<bool>(L*2+5,false));
		rep(j,2){
			rep(k,L*2+5){
				if(dp[j][k]==false)continue;
				
					ndp[0][k+x[i]] = true;
					ndp[1][k+x[i]] = true;
				
					ndp[0][k-x[i]] = true;
					ndp[1][k-x[i]] = true;
				
				
			}
		}
		swap(dp,ndp);
	}
	
	int ans = Inf32;
	rep(i,L*2+5){
		if(dp[0][i])ans = min(ans,abs(i-mid+A));
	}
	
	cout<<ans<<endl;
	return 0;
}