#include <bits/stdc++.h>
using namespace std;
int main() {
  cin.sync_with_stdio(0);
  cin.tie(0);
  cout << fixed << setprecision(10);
  int M, N;
  cin >> M >> N;
  vector<int> X(N);
  for (int i = 0; i < N; i++) {
    cin >> X[i];
    X[i]--;
  }
  if (N == 1) {
    cout << M - 1 << "\n";
    return 0;
  }
  vector<int> L(N);
  for (int i = 0; i < N - 1; i++) L[i] = X[i + 1] - X[i] - 1;
  L[N - 1] = X[0] - 1 + M - X[N - 1];
  int maxt = M, mint = -1;
  while (maxt - mint > 1) {
    int T = (maxt + mint) / 2;
    bool ok = false;
    if (L[0] + 1 <= T)
      for (int k = 0; k < 2; k++) {
        int a = 2, x = 1 + T + X[0], y = X[0] - 1;
        if (k == 0)
          y = min(y, X[1] - T - 1);
        else
          x = max(x, 1 + T + X[1]);
        while (a < N) {
          if (x >= X[a]) {
            x = max(x, X[a] + T + 1);
            a++;
            continue;
          }
          int b = a;
          while (b < N && X[b] - T <= x) b++;
          if (b - a == 0) break;
          if (b - a >= 3) {
            x = X[b - 1] + T + 1;
            a = b;
            continue;
          }
          if (b - a == 2) {
            if (X[b - 1] == X[a] + 1)
              x = X[b - 1] + T + 1;
            else
              x = X[a] + T + 1;
          } else {
            if (X[a] == x)
              x = X[a] + T + 1;
            else
              x = X[a] + 1;
          }
          a = b;
        }
        if (x - y - 1 >= M) ok = true;
      }
    for (int k = 0; k < 1; k++) {
      int a = 1, x = 1 + T + X[0], y = X[0] - 1;
      while (a < N) {
        if (x >= X[a]) {
          x = max(x, X[a] + T + 1);
          a++;
          continue;
        }
        int b = a;
        while (b < N && X[b] - T <= x) b++;
        if (b - a == 0) break;
        if (b - a >= 3) {
          x = X[b - 1] + T + 1;
          a = b;
          continue;
        }
        if (b - a == 2) {
          if (X[b - 1] == X[a] + 1)
            x = X[b - 1] + T + 1;
          else
            x = X[a] + T + 1;
        } else {
          if (X[a] == x)
            x = X[a] + T + 1;
          else
            x = X[a] + 1;
        }
        a = b;
      }
      if (x - y - 1 >= M) ok = true;
    }
    if (L[N - 1] + 1 <= T)
      for (int k = 0; k < 2; k++) {
        int a = 1, x = 1 + X[0], y = X[0] - 1 - T;
        if (k == 0)
          y = X[N - 1] - M - T - 1;
        else
          x = 1 + T + X[N - 1] - M;
        while (a < N - 1) {
          if (x >= X[a]) {
            x = max(x, X[a] + T + 1);
            a++;
            continue;
          }
          int b = a;
          while (b < N - 1 && X[b] - T <= x) b++;
          if (b - a == 0) break;
          if (b - a >= 3) {
            x = X[b - 1] + T + 1;
            a = b;
            continue;
          }
          if (b - a == 2) {
            if (X[b - 1] == X[a] + 1)
              x = X[b - 1] + T + 1;
            else
              x = X[a] + T + 1;
          } else {
            if (X[a] == x)
              x = X[a] + T + 1;
            else
              x = X[a] + 1;
          }
          a = b;
        }
        if (x - y - 1 >= M) ok = true;
      }
    for (int k = 0; k < 1; k++) {
      int a = 1, x = 1 + X[0], y = X[0] - 1 - T;
      while (a < N) {
        if (x >= X[a]) {
          x = max(x, X[a] + T + 1);
          a++;
          continue;
        }
        int b = a;
        while (b < N && X[b] - T <= x) b++;
        if (b - a == 0) break;
        if (b - a >= 3) {
          x = X[b - 1] + T + 1;
          a = b;
          continue;
        }
        if (b - a == 2) {
          if (X[b - 1] == X[a] + 1)
            x = X[b - 1] + T + 1;
          else
            x = X[a] + T + 1;
        } else {
          if (X[a] == x)
            x = X[a] + T + 1;
          else
            x = X[a] + 1;
        }
        a = b;
      }
      if (x - y - 1 >= M) ok = true;
    }
    if (ok)
      maxt = T;
    else
      mint = T;
  }
  cout << maxt << "\n";
  return 0;
}
