#include <iostream>
#include <algorithm>

using namespace std;

typedef long long LL;
typedef pair<int, int> PII;

const int N = 2 * 100010, INF = 0x3f3f3f3f;

int n, l;
PII p[N];

int main() {
    // freopen("water.in", "r", stdin);
    // freopen("water.out", "w", stdout);

    scanf("%d%d", &n, &l);
    for (int i = 0; i < n; ++i) {
        scanf("%d", &p[i].first);
        p[i].second = 1;
    }
    for (int i = 0; i < n; ++i) {
        scanf("%d", &p[n + i].first);
        p[n + i].second = -1;
    }

    sort(p, p + 2 * n);

    LL ma = 0LL;
    int cur = 0, mi = INF, last = -1;
    for (int i = 0; i < 2 * n; ++i) {
        cur += p[i].second;
        if (cur >= ma) {
            ma = cur;
            last = i;
        }
        if (cur < mi) mi = cur;
    }

    int pos = (last + 1) % (2 * n);
    LL ans = 1LL * (ma - mi - (ma > 0 ? 1 : 0)) * l + p[pos].first;

    printf("%lld\n", ans);

    return 0;
}