#include <bits/stdc++.h>
#define fo(a,b,c) for (a=b; a<=c; a++)
#define fd(a,b,c) for (a=b; a>=c; a--)
#define min(a,b) (a<b?a:b)
#define ll long long
//#define file
using namespace std;

int a[100002],b[100002],n,L,i,j,k,l,sum;
ll ans;

void Exit() {printf("-1\n");exit(0);}

int main()
{
	#ifdef file
	freopen("c.in","r",stdin);
	#endif
	
	scanf("%d%d",&n,&L);
	fo(i,1,n) scanf("%d",&a[i]);
	fo(i,1,n) scanf("%d",&b[i]);
	
	a[0]=0,a[n+1]=L+1;
	b[0]=0,b[n+1]=L+1;
	l=0;
	fo(i,0,n)
	if (b[i]+1<b[i+1])
	{
		sum=0;
		while (l<=n && a[l]+1==a[l+1]) ++l;
		k=min(l,i);
		
		while (l<=n && sum<b[i+1]-b[i]-1)
		sum+=a[l+1]-a[l]-1,++l;
		
		if (sum>b[i+1]-b[i]-1) Exit();
		ans+=max(l,i+1)-k-1;
	}
	printf("%lld\n",ans);
	
	fclose(stdin);
	fclose(stdout);
	return 0;
}
#include <bits/stdc++.h>
#define FASTIO
using namespace std;

using ll = long long;
using Vi = std::vector<int>;
using Vl = std::vector<ll>;
using Pii = std::pair<int, int>;
using Pll = std::pair<ll, ll>;

constexpr int I_INF = std::numeric_limits<int>::max();
constexpr ll L_INF = std::numeric_limits<ll>::max();

template <typename T1, typename T2>
inline bool chmin(T1& a, const T2& b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template <typename T1, typename T2>
inline bool chmax(T1& a, const T2& b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

class Prints {
private:
    class __Prints {
    public:
        __Prints(const char* sep, const char* term) : sep(sep), term(term) {}
        template <class... Args>
        void operator()(const Args&... args) const { print(args...); }
        template <typename T>
        void pvec(const T& vec, size_t sz) const {
            for (size_t i = 0; i < sz; i++)
                std::cout << vec[i] << (i == sz - 1 ? term : sep);
        }
        template <typename T>
        void pmat(const T& mat, size_t h, size_t w) {
            for (size_t i = 0; i < h; i++)
                for (size_t j = 0; j < w; j++)
                    std::cout << mat[i][j] << (j == w - 1 ? term : sep);
        }

    private:
        const char *sep, *term;
        void print() const { std::cout << term; }
        void print_rest() const { std::cout << term; }
        template <class T, class... Tail>
        void print(const T& head, const Tail&... tail) const { std::cout << head, print_rest(tail...); }
        template <class T, class... Tail>
        void print_rest(const T& head, const Tail&... tail) const { std::cout << sep << head, print_rest(tail...); }
    };

public:
    Prints() {}
    __Prints operator()(const char* sep = " ", const char* term = "\n") const { return __Prints(sep, term); }
};

Prints prints;

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

void solve() {
    ll N, L;
    cin >> N >> L;
    Vl A(N + 2), B(N + 2);
    for (ll i = 1; i <= N; i++) {
        cin >> A[i];
        A[i] -= i;
    }
    for (ll i = 1; i <= N; i++) {
        cin >> B[i];
        B[i] -= i;
    }
    A[N + 1] = B[N + 1] = (L + 1) - (N + 1);

    Vl pos = B;
    pos.erase(unique(pos.begin(), pos.end()), pos.end());

    ll ans = 0;
    for (auto p : pos) {
        int idxl_a = lower_bound(A.begin(), A.end(), p) - A.begin();
        int idxr_a = upper_bound(A.begin(), A.end(), p) - A.begin();
        int idxl_b = lower_bound(B.begin(), B.end(), p) - B.begin();
        int idxr_b = upper_bound(B.begin(), B.end(), p) - B.begin();
        if (idxr_a - idxl_a == 0) {
            prints()(-1);
            return;
        }
        ans += max(idxr_b - idxr_a, 0) + max(idxl_a - idxl_b, 0);
    }

    prints()(ans);
}

//%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

int main() {
#ifdef FASTIO
    std::cin.tie(nullptr), std::cout.tie(nullptr);
    std::ios::sync_with_stdio(false);
#endif
#ifdef FILEINPUT
    std::ifstream ifs("./in_out/input.txt");
    std::cin.rdbuf(ifs.rdbuf());
#endif
#ifdef FILEOUTPUT
    std::ofstream ofs("./in_out/output.txt");
    std::cout.rdbuf(ofs.rdbuf());
#endif
    std::cout << std::setprecision(18) << std::fixed;
    solve();
    std::cout << std::flush;
    return 0;
}