import std.conv, std.functional, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.container, std.math, std.numeric, std.range, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


int N;
long L;
long[] X, Y;

void main() {
  try {
    for (; ; ) {
      N = readInt();
      L = readLong();
      X = new long[N];
      Y = new long[N];
      foreach (i; 0 .. N) {
        X[i] = readLong();
      }
      foreach (i; 0 .. N) {
        Y[i] = readLong();
      }
      
      auto ps = new Tuple!(long, int)[2 * N];
      foreach (i; 0 .. N) {
        ps[i] = tuple(X[i], +1);
      }
      foreach (i; 0 .. N) {
        ps[N + i] = tuple(Y[i], -1);
      }
      ps.sort;
      auto sum = new int[2 * N + 1];
      foreach (i; 0 .. 2 * N) {
        sum[i + 1] = sum[i] + ps[i][1];
      }
      const mn = sum.minElement;
      const mx = sum.maxElement;
      debug {
        writeln("sum = ", sum);
        writeln("mn = ", mn);
        writeln("mx = ", mx);
      }
      long ans;
      if (mx == 0) {
        foreach (i; 0 .. 2 * N) {
          if (sum[i + 1] == mn) {
            ans = (mx - mn) * L + ps[i][0];
            break;
          }
        }
      } else {
        foreach (i; 0 .. 2 * N) {
          if (sum[i] == mx) {
            ans = (mx - mn - 1) * L + ps[i][0];
          }
        }
      }
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}