#include <iostream>

using namespace std;

typedef long long LL;

const int N = 100010;

struct Event {
    int pos, type;
};

int n, l, x[N], y[N], cur, ma, mi, last, nxt, p, laps;
LL ans;
Event events[N * 2];

int main() {
    scanf("%d%d", &n, &l);
    for (int i = 0; i < n; ++i) scanf("%d", &x[i]);
    for (int i = 0; i < n; ++i) scanf("%d", &y[i]);

    int xi = 0, yi = 0, idx = 0;
    while (xi < n || yi < n) {
        if (xi == n) events[idx++] = { y[yi++], -1 };
        else if (yi == n) events[idx++] = { x[xi++], 1 };
        else if (x[xi] < y[yi]) events[idx++] = { x[xi++], 1 };
        else events[idx++] = { y[yi++], -1 };
    }
    cur = ma = 0, mi = n;
    last = -1;
    for (int i = 0; i < n * 2; ++i) {
        cur += events[i].type;
        if (cur >= ma) {
            ma = cur;
            last = i;
        }
        if (cur <= mi) mi = cur;
    }
    nxt = (last + 1) % (n * 2);
    p = events[nxt].pos;
    laps = ma - mi - (ma > 0 ? 1 : 0);
    ans = 1LL * laps * l + p;
    printf("%lld\n", ans);

    return 0;
}