#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 5;
long long n, m, l, r, a[N], b[N], dp[N];
void prework() {
  int k = 1;
  for (int i = 2; i <= n; ++i)
    if (a[i + 1] - a[i] > a[k + 1] - a[k]) k = i;
  r = a[k + 1] - a[k] - 1;
  for (int i = 1; i <= n; ++i) b[i] = a[i + k];
  int t = b[1];
  for (int i = 1; i <= n; ++i) b[i] -= t;
}
int check(long long x) {
  dp[1] = 0;
  for (int i = 2; i <= n; ++i) {
    dp[i] = dp[i - 1];
    if (dp[i - 1] >= b[i] - 1) dp[i] = max(dp[i], b[i] + x);
    if (dp[i - 1] >= b[i] - x - 1) dp[i] = max(dp[i], b[i]);
    if (i > 2 && dp[i - 2] >= b[i] - x - 1) dp[i] = max(dp[i], b[i - 1] + x);
  }
  if (dp[n] >= m - x - 1) return 1;
  dp[2] = max(b[2], x);
  for (int i = 3; i <= n; ++i) {
    dp[i] = dp[i - 1];
    if (dp[i - 1] >= b[i] - 1) dp[i] = max(dp[i], b[i] + x);
    if (dp[i - 1] >= b[i] - x - 1) dp[i] = max(dp[i], b[i]);
    if (dp[i - 2] >= b[i] - x - 1) dp[i] = max(dp[i], b[i - 1] + x);
  }
  return dp[n] >= min(m - 1, m + b[2] - x - 1);
}
int main() {
  ios::sync_with_stdio(false);
  cin >> m >> n;
  for (int i = 1; i <= n; ++i) cin >> a[i], a[i + n] = a[i] + m;
  if (n == 1) return cout << m - 1 << endl, 0;
  sort(a + 1, a + n * 2 - 1);
  prework();
  while (l < r) {
    long long mid = (l + r) >> 1;
    if (check(mid))
      r = mid;
    else
      l = mid + 1;
  }
  cout << l << endl;
  return 0;
}
