#include <bits/stdc++.h>
using namespace std;
 
typedef long long ll;
typedef long double ld;
typedef unsigned int uint;
typedef vector<int> vi;
typedef vector< vector <int> > vvi;
typedef pair<int, int> pii;
typedef pair < pair < int, int >, int > piii;
typedef pair < pair <int, int > , pair <int, int> > piiii;
typedef pair<ll, ll> pll;
typedef vector<bool> vb;
typedef vector<char> vc;
typedef vector<string> vs;
 
#define FOR(i,a,b) for(int i = a; i < b; i ++)
#define RFOR(i,a,b) for(int i = a-1; i >= b; i --)
#define all(a) a.begin(), a.end()
#define endl '\n';
#define sz(x) (int)(x).size()
 
#define mp make_pair
#define pb push_back
#define ff first
#define ss second
 
template <typename T>
void pr(vector<T> &v) {
    FOR(i, 0, sz(v)) cout << v[i] << " ";
    cout << endl;
}
template <typename T>
void pr(vector<vector<T> > &v) {
    FOR(i, 0, sz(v)) { pr(v[i]); }
}
template <typename T>
void re(T &x) {
    cin >> x;
}
template <typename T>
void re(vector<T> &a) {
    FOR(i, 0, sz(a)) re(a[i]);
}
template <class Arg, class... Args>
void re(Arg &first, Args &... rest) {
    re(first);
    re(rest...);
}
template <typename T>
void pr(T x) {
    cout << x << endl;
}
template <class Arg, class... Args>
void pr(const Arg &first, const Args &... rest) {
    cout << first << " ";
    pr(rest...);
    cout << endl;
}
void ps() { cout << endl; }
template<class T, class... Ts>
void ps(const T& t, const Ts&... ts) {
    cout << t; if (sizeof...(ts)) cout << " "; ps(ts...);
}
 
const ll MOD  =  998244353;
#define inf 1e18;
#define INF INT_MAX

long double PI = 4*atan(1);
long double eps = 1e-12;


int main() {
    //auto start = chrono::high_resolution_clock::now();
    ios_base::sync_with_stdio(0);cin.tie(0);
    // ofstream cout("disrupt.out");
    // ifstream cin("disrupt.in");
    #ifdef DEBUG
      freopen("input.txt", "r", stdin);
      freopen("output.txt", "w", stdout);
    #endif 

    int b,p; cin >> b >> p;
    vector<int> x(p); re(x);
    reverse(all(x));
    x.pb(0);
    vi y(p+1);
    bool good = false;
    FOR(i,0,p) {
        if(!good) {
            if(x[i] == 0) y[i] = 0;
            else y[i] = b - x[i];
        } else {
            y[i] = b-1 - x[i];
        }
        if(x[i] != 0) good = true;
    }

    vi xlo = x, yhi = y;
    vi xhi = x, ylo = y;

    // pr(x);
    // pr(y);
    // cout << "***" << endl;
    
    int dp[2][100000] = {};
    memset(dp, 0x3f, sizeof(dp));
    dp[0][0] = 0;
    dp[1][0] = 0;

    FOR(t,0,p) {
        int dp2[2][100000] = {};
        memset(dp2, 0x3f, sizeof(dp2));
        //FOR(k,0,2) FOR(j,0,100000) {dp2[k][j] = dp[k][j];}
    
        FOR(i,0,100000) {
            if(dp[0][i] < 1e8) {
                dp2[0][i] = min(dp2[0][i], dp[0][i] + ylo[t]);
                dp2[1][i + xhi[t]] = min(dp2[1][i + xhi[t]], dp[0][i]);
            }
            if(dp[1][i] < 1e8) {
                dp2[0][i] = min(dp2[0][i], dp[1][i] + yhi[t]);
                dp2[1][i + xlo[t]] = min(dp2[1][i + xlo[t]], dp[1][i]);
            }
        }
        
        FOR(k,0,2) FOR(j,0,100000) {dp[k][j] = dp2[k][j];}

        // FOR(k,0,2) {
        //     FOR(j,0,20) {
        //         cout << dp[k][j] << " ";
        //     }
        //     cout << endl;
        // }
        yhi[t] += xlo[t];
        xlo[t] = 0;
        int ptr = t;
        while(yhi[ptr] == b) {
            yhi[ptr] = 0;
            yhi[ptr+1]++;
            ptr++;
        }
        // pr(xlo);
        // pr(yhi);
      //  cout << "----" << endl;

        xhi[t] += ylo[t];
        ylo[t] = 0;
        ptr = t;
        while(xhi[ptr] == b) {
            xhi[ptr] = 0;
            xhi[ptr+1]++;
            ptr++;
        }
        // pr(xhi);
        // pr(ylo);
        // cout << "-----------" << endl;
    }

    int ans = 1e9;
    FOR(i,0,2) {
        FOR(j,0,100000) {
            if(dp[i][j] < 1e8) {
                ans = min(ans, max(j, dp[i][j]));
            }
        }
    }
    cout << ans << endl;

    // cout << 10 << " " << 1000 << endl;
    // FOR(i,0,1000) {
    //     cout << << " ";
    // }
    // cout << endl;





    

    // auto stop = chrono::high_resolution_clock::now();
    // auto duration = chrono::duration_cast<chrono::microseconds>(stop - start);
    // cout << duration.count() << endl;
    //cin.close();
    //cout.close();
}