#include <bits/stdc++.h>
using namespace std;
void debug() { cout << endl; }
template <class T, class... R>
void debug(T f, R... r) {
  cout << "[" << f << "]";
  debug(r...);
}
template <class T>
T mmin(T a, T b) {
  return min(a, b);
}
template <class T, class... R>
T mmin(T a, R... b) {
  return min(a, mmin(b...));
}
template <class T>
T mmax(T a, T b) {
  return max(a, b);
}
template <class T, class... R>
T mmax(T a, R... b) {
  return max(a, mmax(b...));
}
template <class T>
T mgcd(T a, T b) {
  return __gcd(a, b);
}
template <class T, class... R>
T mgcd(T a, R... b) {
  return __gcd(a, mgcd(b...));
}
void myin() {
  freopen("1.in", "r", stdin);
  freopen("1.out", "w", stdout);
}
namespace IO {
const int MX = 4e7;
char buf[MX];
int c, sz;
void begin() {
  c = 0;
  sz = fread(buf, 1, MX, stdin);
}
inline bool read(int &t) {
  while (c < sz && buf[c] != '-' && (buf[c] < '0' || buf[c] > '9')) c++;
  if (c >= sz) return false;
  bool flag = 0;
  if (buf[c] == '-') flag = 1, c++;
  for (t = 0; c < sz && '0' <= buf[c] && buf[c] <= '9'; c++)
    t = t * 10 + buf[c] - '0';
  if (flag) t = -t;
  return true;
}
}  // namespace IO
const int N = 1e5 + 5;
const int M = 1e5 + 5;
int n, m;
int a[300005];
int b[300005];
int f(int mid) {
  for (int i = 1; i <= n; ++i) b[i] = a[i];
  for (int i = 1; i <= n; ++i) {
    if ((b[i] + mid) % m >= b[i - 1] && b[i] + mid >= m)
      b[i] = b[i - 1];
    else if (b[i] >= b[i - 1])
      b[i] = b[i];
    else if (b[i] + mid >= b[i - 1])
      b[i] = b[i - 1];
    else
      return 0;
  }
  return 1;
}
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= n; ++i) scanf("%d", &a[i]);
  int l = 0, r = m, ans, mid;
  while (l <= r) {
    mid = l + r >> 1;
    if (f(mid))
      ans = mid, r = mid - 1;
    else
      l = mid + 1;
  }
  printf("%d\n", ans);
  return 0;
}
