#include <bits/stdc++.h>
using namespace std;
using ll=long long;
#define fi first
#define se second
#define rep(i,n) for(int i=0;i<n;i++)
#define all(v) v.begin(),v.end()
#define pb push_back
template<class T,class U>
inline bool chmax(T &a,U b){
    if(a<b){
        a=b;
        return true;
    }
    return false;
}
template<class T,class U>
inline bool chmin(T &a,U b){
    if(a>b){
        a=b;
        return true;
    }
    return false;
}
constexpr int INF=1000000000;
constexpr ll llINF=3000000000000000000;
constexpr int mod=1000000007;
constexpr double eps=1e-8;
const double pi=acos(-1);
int dx[]={0,1,0,-1},dy[]={1,0,-1,0};
int Random(int mi,int ma){
    random_device rnd;
    mt19937 mt(rnd());//32bit
    //[mi,ma]
    uniform_int_distribution<int>engine(mi,ma);
    return engine(mt);
}
/*
vector<vector<ll>>C,sC;
void init_comb(int n,int m){
    C.resize(n+1,vector<ll>(m+1,0));
    sC.resize(n+1,vector<ll>(m+1,0));
    C[0][0]=1;
    for(int i=1;i<=n;i++){
        C[i][0]=1;
        for(int j=1;j<=m;j++){
            C[i][j]=(C[i-1][j-1]+C[i-1][j])%mod;
        }
    }
    rep(i,n+1){
        rep(j,m){
            sC[i][j+1]=(sC[i][j]+C[i][j])%mod;
        }
    }
}*/
ll gcd(ll a,ll b){
    while(a%b){
        a%=b;
        swap(a,b);
    }
    return b;
}
ll lcm(ll a,ll b){
    return a/gcd(a,b)*b;
}
bool isprime(int a){
    if(a==1)return false;
    for(int i=2;i*i<=a;i++){
        if(a%i==0)return false;
    }
    return true;
}
vector<int>primes;
void init_prime(int n){
	primes.push_back(2);
    for(int i=3;i<=n;i+=2){
        bool f=true;
        for(int j:primes){
            if(j*j>i)break;
            if(i%j==0){
                f=false;
                break;
            }
        }
        if(f)primes.push_back(i);
    }
}
ll modpow(ll a,ll b){
    ll res=1;
    while(b){
        if(b&1){
            res*=a;
            res%=mod;
        }
        a*=a;
        a%=mod;
        b>>=1;
    }
    return res;
}
vector<ll>inv,fact,factinv;
void init_fact(int n){
    inv.resize(n+1);
    fact.resize(n+1);
    factinv.resize(n+1);
    inv[0]=0;
    inv[1]=1;
    fact[0]=1;
    factinv[0]=1;
    for(ll i=1;i<=n;i++){
        if(i>=2)inv[i]=mod-((mod/i)*inv[mod%i]%mod);
        fact[i]=(fact[i-1]*i)%mod;
        factinv[i]=factinv[i-1]*inv[i]%mod;
    }
}
ll _inv(ll a,ll m=mod){
    //gcd(a,m) must be 1
    ll b=m,u=1,v=0;
    while(b){
        ll t=a/b;
        a-=t*b;swap(a,b);
        u-=t*v;swap(u,v);
    }
    u%=m;
    if(u<0)u+=m;
    return u;
}
ll comb(int a,int b){
    if(a<b)return 0;
    if(a<0)return 0;
    return fact[a]*factinv[a-b]%mod*factinv[b]%mod;
}
ll multicomb(int a,int b){
    return comb(a+b-1,b);
}
int n,m;
vector<int>g[1010][26];
int dp[1010][1010];
#define P pair<int,int>
#define PP pair<int,P>
void solve(){
    cin>>n>>m;
    priority_queue<PP,vector<PP>,greater<PP>>q;
    rep(i,n)rep(j,n)dp[i][j]=INF;
    rep(i,n){
        dp[i][i]=0;
        q.push({0,{i,i}});
    }
    rep(i,m){
        int a,b;
        char c;
        cin>>a>>b>>c;
        a--,b--;
        g[a][c-'a'].push_back(b);
        g[b][c-'a'].push_back(a);
        if(a!=b){
            dp[a][b]=dp[b][a]=1;
            q.push({1,{a,b}});
            q.push({1,{b,a}});
        }
    }
    while(!q.empty()){
        PP p=q.top();
        q.pop();
        int l=p.second.first,r=p.second.second;
        if(dp[l][r]!=p.first)continue;
        rep(k,26){
            for(int a:g[l][k]){
                for(int b:g[r][k]){
                    if(chmin(dp[a][b],p.first+2)){
                        q.push({dp[a][b],{a,b}});
                    }
                }
            }
        }
    }
    cout<<(dp[0][n-1]==INF?-1:dp[0][n-1])<<endl;
}
int main(){
    cin.tie(0);ios::sync_with_stdio(false);
    //int t;cin>>t;while(t--)
    solve();
    return 0;
}
#include <bits/stdc++.h>

using namespace std;

const int kmax = 1003;

struct Node {
  int x, y;
} q[kmax * kmax];

struct O {
  int to;
  char c;
} o[kmax];

int ans = 1e9;
bool l[kmax][kmax];
int n, m, d[kmax][kmax], h = 1, t;
vector<O> p[kmax];
char c;

void E(int x, int y, int z) {
  if (d[x][y]) {
    return;
  }
  d[x][y] = d[y][x] = z;
  q[++t] = {x, y};
  if (x == y) {
    ans = min(ans, z);
  } else if (l[x][y]) {
    ans = min(ans, z + 1);
  }
}

int main() {
  cin >> n >> m;
  for (int i = 1, x, y; i <= m; i++) {
    cin >> x >> y >> c;
    p[x].push_back({y, c});
    p[y].push_back({x, c});
    l[x][y] = l[y][x] = 1;
  }
  for (E(1, n, 1); h <= t; h++) {
    for (int i = 0; i < p[q[h].x].size(); i++) {
      for (int j = 0; j < p[q[h].y].size(); j++) {
        if (p[q[h].x][i].c == p[q[h].y][j].c) {
          E(p[q[h].x][i].to, p[q[h].y][j].to, d[q[h].x][q[h].y] + 2);
        }
      }
    }
  }
  cout << (ans == 1e9 ? -1 : ans - 1);
  return 0;
}
