#include <iostream>
using namespace std;

int main () {
    int n, L; cin >> n >> L;
    int x[n], y[n];
    for (int i = 0; i < n; i++) cin >> x[i];
    for (int i = 0; i < n; i++) cin >> y[i];
    int i = 0, j = 0, prev = 0, count = 0, pos = 0;
    long long ans = 0;
    int consecCount[2] = {0, 0};
    while (i < n || j < n) {
        if (j == n) {
            if (prev == 1) count = 0, pos = x[i];
            count += n - i;
            i = n;
        } else if (i == n) {
            if (prev == 0) count = 0, pos = y[j];
            count += n - j;
            j = n;
        } else if (x[i] < y[j]) {
            if (prev == 0) count++;
            else {
                prev = 0;
                count = 1;
                pos = x[i];
            }
            i++;
        } else {
            if (prev == 1) count++;
            else {
                prev = 1; count = 1;
                pos = y[j];
            }
            j++;
        }
        consecCount[prev] = max(consecCount[prev], count);
        int loops = count;
        if (prev == 1 && consecCount[0] < count) loops++;
        ans = max(ans, (long long)(loops - 1) * L + pos);
    }
    cout << ans << endl;
}