#include <bits/stdc++.h>
using namespace std;
int m, n;
int a[200000 + 5], p[200000 + 5];
int dp[200000 + 5];
bool check(int x) {
  for (int i = 1; i <= n; i++) dp[i] = 0;
  for (int s = 1; s <= 2; s++) {
    if (s == 1)
      dp[s] = 0;
    else
      dp[s] = max(p[s], x);
    for (int i = s + 1; i <= n; i++) {
      dp[i] = dp[i - 1];
      if (dp[i - 1] >= p[i] - 1) dp[i] = p[i] + x;
      if (dp[i - 1] >= p[i] - x - 1) dp[i] = max(dp[i], p[i]);
      if (i - 2 >= s && dp[i - 2] >= p[i] - x - 1)
        dp[i] = max(dp[i], p[i - 1] + x);
    }
    if (dp[n] >= min(m + p[s] - x - 1, m - 1)) return 1;
  }
  return 0;
}
int main() {
  scanf("%d %d", &m, &n);
  for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
  sort(a + 1, a + 1 + n);
  for (int i = 1; i <= n; i++) a[i + n] = a[i] + m;
  int md = 0;
  for (int i = 1; i <= n; i++) md = max(md, a[i + 1] - a[i]);
  for (int i = 1; i <= n; i++) {
    if (a[i + 1] - a[i] == md) {
      for (int j = 1; j <= n; j++) p[j] = a[i + j];
      for (int j = n; j >= 1; j--) p[j] -= p[1];
      break;
    }
  }
  int l = 0, r = md, ans = 0, mid;
  while (l <= r) {
    mid = (l + r) >> 1;
    if (check(mid)) {
      ans = mid;
      r = mid - 1;
    } else
      l = mid + 1;
  }
  printf("%d\n", ans);
}
