#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> pll;
typedef pair<ll, ll> PLL;
#define endl '\n'
#define rep(i, a, b) for(ll i = (a); i <= (b); i++)
#define per(i, a, b) for(ll i = (a); i >= (b); i--)
#define debug(x) cout<<#x<<": "<<x<<endl
const ll mod=998244353;
const int N=2e5+10;
typedef pair<int,int>PII;

int n;
vector<int> e[N],tp;//e为终点,tp为路径
int c[N];

bool dfs(int x)
{
    c[x]=-1;
    for(auto p:e[x])
    {
        //dfn[p]=max(dfn[p],dfn[x]+1);
        if(c[p]<0) return 0;
        else if(!c[p])
        {
            if(!dfs(p)) return 0;
        }
    }
    c[x]=1;
    tp.push_back(x);
    return 1;
}

bool toposort()
{
    for(int i=1;i<=n;i++)
    {
        if(!c[i])
        {
            //dfn[i]=0;
            if(!dfs(i)) return 0;
        }
    }
    reverse(tp.begin(),tp.end());
    return 1;
}

void solve()
{
    cin>>n;
    int m;
    cin>>m;
    vector<PII>path[n+1];
    ll num[n+1];
    memset(num,0,sizeof(num));
    for(int i=1;i<=m;i++)
    {
        int x,y,z;
        cin>>x>>y>>z;
        e[x].push_back(z);
        e[y].push_back(z);
        path[z].push_back({x,y});
    }
    if(!toposort())
    {
        cout<<-1<<endl;
        return;
    }
    ll sum=0;
    for(auto p:tp)
    {
        for(auto [x,y]:path[p])
        {
            num[p]=max(num[p],num[x]+num[y]);
        }
        if(num[p]==0) num[p]=1;
        sum+=num[p];
        if(sum>1e9)
        {
            cout<<-1<<endl;
            return;
        }
    }
    cout<<sum<<endl;
}

int main()
{
    // #ifndef ONLINE_JUDGE
    //     freopen("test.in","r",stdin);
    //     freopen("test.out","w",stdout);
    // #endif
    ios::sync_with_stdio(0);
    cin.tie(0),cout.tie(0);
    int T=1;
    //cin>>T;
    while(T--)
    {
        solve();
    }
    return 0;
}