#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define P pair<int,char>
#define Tp tuple<int,int,int>
using Graph = vector<vector<P>>;

int main(){
    int N,M; cin >> N >> M;
    int a,b; char c;
    Graph g(N+1);
    rep(i,M){
        cin >> a >> b >> c;
        g[a].push_back(make_pair(b,c));
        g[b].push_back(make_pair(a,c));
    }
    queue<tuple<int,int,int>> go;
    bool vis[N+1][N+1];
    rep(i,N+1){
        rep(j,N+1){
            vis[i][j] = false;
        }
    }
    vis[1][N] = true; go.push(make_tuple(0,1,N));
    int nowk = 0; bool fin = false;
    while(!go.empty()){
        auto p = go.front(); go.pop();
        a = get<1>(p); b = get<2>(p);
        int k = get<0>(p);
        if(k>nowk){
            nowk = k;
            if(fin){
                cout << 2*k-1 << endl; return 0;
            }
        }
        if(a==b){
            cout << 2*k << endl; return 0;
        }
        for(auto q:g[a]){
            int x = q.first; char c1 = q.second; int y;
            for(auto r:g[b]){
                y = r.first; char c2 = r.second;
                if(x==b&&y==a){
                    fin = true;
                }
                if(c1!=c2||vis[x][y]) continue;
                go.push(make_tuple(k+1,x,y)); vis[x][y] = true;
            }
        }
    }
    cout << -1 << endl;
}
#include <bits/stdc++.h>

using namespace std;


using int64 = long long;
const int mod = 998244353;

const int64 infll = (1LL << 62) - 1;
const int inf = (1 << 30) - 1;

struct IoSetup {
  IoSetup() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
    cerr << fixed << setprecision(10);
  }
} iosetup;


template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p) {
  os << p.first << " " << p.second;
  return os;
}

template<typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p) {
  is >> p.first >> p.second;
  return is;
}

template<typename T>
ostream &operator<<(ostream &os, const vector<T> &v) {
  for (int i = 0; i < (int) v.size(); i++) {
    os << v[i] << (i + 1 != v.size() ? " " : "");
  }
  return os;
}

template<typename T>
istream &operator>>(istream &is, vector<T> &v) {
  for (T &in : v) is >> in;
  return is;
}

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

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

template<typename T = int64>
vector<T> make_v(size_t a) {
  return vector<T>(a);
}

template<typename T, typename... Ts>
auto make_v(size_t a, Ts... ts) {
  return vector<decltype(make_v<T>(ts...))>(a, make_v<T>(ts...));
}

template<typename T, typename V>
typename enable_if<is_class<T>::value == 0>::type fill_v(T &t, const V &v) {
  t = v;
}

template<typename T, typename V>
typename enable_if<is_class<T>::value != 0>::type fill_v(T &t, const V &v) {
  for (auto &e : t) fill_v(e, v);
}

template<typename F>
struct FixPoint : F {
  FixPoint(F &&f) : F(forward<F>(f)) {}

  template<typename... Args>
  decltype(auto) operator()(Args &&... args) const {
    return F::operator()(*this, forward<Args>(args)...);
  }
};

template<typename F>
inline decltype(auto) MFP(F &&f) {
  return FixPoint<F>{forward<F>(f)};
}


int main() {
  int N, L;
  cin >> N >> L;
  vector< int > A(N + 2), B(N + 2);
  for(int i = 1; i <= N; i++) cin >> A[i];
  for(int i = 1; i <= N; i++) cin >> B[i];
  A[N + 1] = L + 1;
  B[N + 1] = L + 1;
  for(int i = 0; i < N + 2; i++) A[i] -= i;
  for(int i = 0; i < N + 2; i++) B[i] -= i;

  map< int, int > ls, rs;
  for(int i = 0, j; i < N + 2; i = j) {
    for(j = i; j < N + 2 and A[i] == A[j]; j++) ;
    ls[A[i]] = i;
    rs[A[i]] = j;
  }
  int64 ret = 0;
  for(int i = 0, j; i < N + 2; i = j) {
    for(j = i; j < N + 2 and B[i] == B[j]; j++) ;
    if(!ls.count(B[i])) {
      cout << -1 << endl;
      exit(0);
    }
    ret += max(ls[B[i]] - i, 0);
    ret += max(j - rs[B[i]], 0);
  }
  cout << ret << endl;
}
