#include <stdio.h>

int main () {
  int n = 0;
  int m = 0;
  int a[200000] = {};
  int b = 0;
  
  int res = 0;
  
  long long ans = 0LL;
  long long dp[200000][2] = {};
  
  res = scanf("%d", &n);
  res = scanf("%d", &m);
  for (int i = 0; i < n; i++) {
    res = scanf("%d", a+i);
  }
  for (int i = 0; i < n; i++) {
    res = scanf("%d", &b);
    a[i] = (b-a[i]+m)%m;
  }
  
  dp[0][0] = (long long)a[0];
  dp[0][1] = (long long)(m-a[0]);
  for (int i = 1; i < n; i++) {
    if (a[i-1] < a[i]) {
      dp[i][0] = dp[i-1][0]+((long long)(a[i]-a[i-1]));
      dp[i][1] = dp[i-1][1]+((long long)(a[i]-a[i-1]));
    } else {
      dp[i][0] = dp[i-1][0]-((long long)(a[i]-a[i-1]));
      dp[i][1] = dp[i-1][1]-((long long)(a[i]-a[i-1]));
    }
    if (dp[i][0] > dp[i-1][1]+((long long)(a[i]+m-a[i-1]))) {
      dp[i][0] = dp[i-1][1]+((long long)(a[i]+m-a[i-1]));
    }
    if (dp[i][1] > dp[i-1][0]+((long long)(a[i-1]+m-a[i]))) {
      dp[i][1] = dp[i-1][0]+((long long)(a[i-1]+m-a[i]));
    }
  }
  
  ans = dp[n-1][0]+((long long)a[n-1]);
  if (ans > dp[n-1][1]+((long long)(m-a[n-1]))) {
    ans = dp[n-1][1]+((long long)(m-a[n-1]));
  }
  
  printf("%lld\n", ans/2LL);
  return 0;
}