// marupakuri of: https://yukicoder.me/submissions/327798

#pragma GCC optimize("Ofast")
#pragma GCC target("avx2")

char*mmap();
#define RD(v) int v=0;{int _c;while(_c=*rp++-48,_c>=0)v=v*10+_c;}

int x[100001],y[100001];

int main() {
	char*rp=mmap(0l,1l<<28,1,2,0,0ll);
	RD(N);
	RD(L);
	for (int i = 0;i != N;++i) {
		RD(xi); x[i]=xi;
	}
	x[N] = L;
	for (int i = 0;i != N;++i) {
		RD(yi); y[i]=yi;
	}
	y[N] = L;

	int count = 0, max = 0, min = 0;
	int *xitr = x, *yitr = y;
	int last;
	for (int i = 0;i != N * 2;++i) {
		if (*xitr < *yitr) {
			++count;
			if (max <= count) {
				max = count;
				last = *yitr;
			}
			++xitr;
		}
		else {
			--count;
			if (count <= min) {
				min = count;
			}
			++yitr;
		}
	}
	int diff = max - min;
	long ans;
	if (max == 0) {
		ans = (long)L*diff + y[0];
	}
	else {
		ans = (long)L*diff - (L - last);
	}
	printf("%ld\n", ans);
	return 0;
}