//#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,sse4.1,sse4.2,popcnt,abm,mmx,avx,avx2,fma,tune=native") 

#include <bits/stdc++.h>

using namespace std ;

#define int int64_t //be careful about this 
#define endl "\n"
#define f(i,a,b) for(int i=int(a);i<int(b);++i)

#define pr pair
#define ar array
#define fr first
#define sc second
#define vt vector
#define pb push_back
#define LB lower_bound  
#define UB upper_bound
#define PQ priority_queue

#define sz(x) ((int)(x).size())
#define all(a) (a).begin(),(a).end()
#define allr(a) (a).rbegin(),(a).rend()
#define mem0(a) memset(a, 0, sizeof(a))
#define mem1(a) memset(a, -1, sizeof(a))

template<class A> void rd(vt<A>& v);
template<class T> void rd(T& x){ cin >> x; }
template<class H, class... T> void rd(H& h, T&... t) { rd(h) ; rd(t...) ;}
template<class A> void rd(vt<A>& x) { for(auto& a : x) rd(a) ;}

template<class T> bool ckmin(T& a, const T& b) { return b < a ? a = b, 1 : 0; }
template<class T> bool ckmax(T& a, const T& b) { return a < b ? a = b, 1 : 0; }

template<typename T>
void __p(T a) {
  cout<<a; 
}
template<typename T, typename F>
void __p(pair<T, F> a) {
  cout<<"{";
  __p(a.first);
  cout<<",";
  __p(a.second);
  cout<<"}\n"; 
}
template<typename T>
void __p(std::vector<T> a) {
  cout<<"{";
  for(auto it=a.begin(); it<a.end(); it++)
    __p(*it),cout<<",}\n"[it+1==a.end()]; 
}
template<typename T, typename ...Arg>
void __p(T a1, Arg ...a) {
  __p(a1);
  __p(a...);
}
template<typename Arg1>
void __f(const char *name, Arg1 &&arg1) {
  cout<<name<<" : ";
  __p(arg1);
  cout<<endl;
}
template<typename Arg1, typename ... Args>
void __f(const char *names, Arg1 &&arg1, Args &&... args) {
  int bracket=0,i=0;
  for(;; i++)
    if(names[i]==','&&bracket==0)
      break;
    else if(names[i]=='(')
      bracket++;
    else if(names[i]==')')
      bracket--;
  const char *comma=names+i;
  cout.write(names,comma-names)<<" : ";
  __p(arg1);
  cout<<" | ";
  __f(comma+1,args...);
}

void setIO(string s = "") {
  ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0); 
  cin.exceptions(cin.failbit); 
	cout.precision(15);	cout << fixed;
  #ifdef ONLINE_JUDGE
  if(sz(s)){
  	freopen((s+".in").c_str(),"r",stdin);
  	freopen((s+".out").c_str(),"w",stdout);
  }
  #endif
}
const int inf = 1e9, A = 26;
signed main(){
  setIO() ;
  int n,m; rd(n,m);
  int dp[n][n];  
  f(i,0,n) f(j,0,n) dp[i][j] = inf ;
  f(i,0,n) dp[i][i] = 0;
  vt<int> g[n][A];
  f(_,0,m){
  	int u,v; char c;
  	rd(u,v,c); 
  	--u,--v;
  	ckmin(dp[u][v],int(1));
  	ckmin(dp[v][u],int(1));
  	g[u][c-'a'].pb(v);
  	g[v][c-'a'].pb(u);
  }
  PQ<ar<int,3>> q;
  f(i,0,n) f(j,0,n) q.push({-dp[i][j],i,j});
  while(sz(q)){
  	auto a = q.top(); q.pop();
  	int v = -a[0], i = a[1], j = a[2];
  	if(dp[i][j] != v) continue;
  	f(k,0,A){
  		for(auto x : g[i][k]){
  			for(auto y : g[j][k]){
  				if(ckmin(dp[x][y],v + 2)){
  					q.push({-dp[x][y],x,y});
  				}
  			}
  		} 
  	}
  }
  cout << (dp[0][n-1] == inf ? int(-1) : dp[0][n-1]);
}


#include <bits/stdc++.h>
using namespace std;

const int inf = 1 << 29;
const int N = 1100;
vector<int> edge[N][26];
vector<int> edge2[N * N];
int dist[N * N];

void bfs(int s)
{
    queue<int> q;
    fill(dist, dist + N * N, inf);
    q.push(s);
    dist[s] = 0;
    while (q.size())
    {
        int v = q.front();
        q.pop();
        for (auto to : edge2[v])
        {
            if (dist[to] != inf)
                continue;
            dist[to] = dist[v] + 1;
            q.push(to);
        }
    }
}

int main()
{
    int n, m;
    cin >> n >> m;
    for (int i = 0; i < m; i++)
    {
        int a, b;
        char c;
        cin >> a >> b >> c;
        a--, b--;
        c -= 'a';
        edge[a][c].push_back(b);
        edge[b][c].push_back(a);
    }

    for (int i = 0; i < n; i++)
        for (int j = 0; j < n; j++)
        {
            int from = i * N + j;
            for (int c = 0; c < 26; c++)
                for (int ito : edge[i][c])
                    for (int jto : edge[j][c])
                    {
                        int to = ito * N + jto;
                        edge2[from].push_back(to);
                    }
        }

    int start = 0 * N + (n - 1);
    bfs(start);

    int ans = inf;
    for (int i = 0; i < n; i++)
    {
        ans = min(ans, dist[i * N + i] * 2);
        for (int c = 0; c < 26; c++)
            for (int to : edge[i][c])
                ans = min(ans, dist[i * N + to] * 2 + 1);
    }

    if (ans == inf)
        ans = -1;
    cout << ans << endl;
}
