#include <bits/stdc++.h>
using namespace std;
long long int dx[] = {1, -1, 0, 0};
long long int dy[] = {0, 0, -1, 1};
long long int gcd(long long int a, long long int b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
long long int power_mod(long long int a, long long int n) {
  long long int res = 1;
  while (n > 0) {
    if (n & 1)
      res = ((res % 1000000007) * (a % 1000000007)) % 1000000007, n--;
    else
      a = ((a % 1000000007) * (a % 1000000007)) % 1000000007, n /= 2;
  }
  return res;
}
long long int nCr(long long int n, long long int r) {
  if (r > n) return 0;
  double res = 1;
  for (long long int i = 1; i <= r; i++) res = (res * (n - r + i)) / i;
  return (long long int)(res + 0.01);
}
long long int power(long long int a, long long int n) {
  long long int res = 1;
  for (long long int i = 0; i < n; i++) res = res * a;
  return res;
}
bool cmp(vector<long long int> a, vector<long long int> b) {
  return a[0] < b[0];
}
bool is_power_of_two(long long int n) {
  for (long long int i = 0; i < 32; i++) {
    if ((1 << i) == n) return true;
  }
  return false;
}
bool check(long long int x, long long int a[], long long int n,
           long long int m) {
  long long int b[n];
  for (long long int i = 0; i < n; i++) b[i] = a[i];
  long long int r = m - b[0];
  if (r <= x) b[0] = 0;
  for (long long int i = 1; i < n; i++) {
    if (b[i] > b[i - 1]) {
      long long int p = m - b[i] + b[i - 1];
      if (p <= x) b[i] = b[i - 1];
    } else if (b[i] == b[i - 1])
      continue;
    else {
      long long int p = b[i - 1] - b[i];
      if (p <= x)
        b[i] = b[i - 1];
      else
        return false;
    }
  }
  return true;
}
void solve() {
  long long int n, m;
  cin >> n >> m;
  long long int a[n];
  for (long long int i = 0; i < n; i++) cin >> a[i];
  long long int l = 0, r = m;
  long long int ans;
  while (l <= r) {
    long long int mid = l + (r - l) / 2;
    if (check(mid, a, n, m)) {
      ans = mid;
      r = mid - 1;
    } else {
      l = mid + 1;
    }
  }
  cout << ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long int t = 1;
  while (t--) {
    solve();
  }
}
