import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, 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; }

string COLOR(string s = "") { return "\x1b[" ~ s ~ "m"; }

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)); }




void main() {
  try {
    for (; ; ) {
      const N = readInt;
      const M = readLong;
      auto A = new long[N + 2]; foreach (i; 1 .. N + 1) A[i] = readLong;
      auto B = new long[N + 2]; foreach (i; 1 .. N + 1) B[i] = readLong;
      
      auto C = new long[N + 2];
      foreach (i; 1 .. N + 1) {
        C[i] = (A[i] - B[i]) % M;
        if (C[i] < 0) C[i] += M;
      }
      auto D = new long[N + 1];
      foreach (i; 0 .. N + 1) {
        D[i] = C[i + 1] - C[i];
        if (D[i] < 0) D[i] += M;
      }
      const sumD = D.sum;
      debug {
        writeln("D = ", D);
        writeln("sumD = ", sumD);
      }
      
      auto costs = new long[N + 1];
      foreach (i; 0 .. N + 1) {
        costs[i] = (M - D[i]) - D[i];
      }
      costs.sort;
      
      long ans;
      ans += sumD;
      ans += costs[0 .. sumD / M].sum;
      ans /= 2;
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}