#include<bits/stdc++.h>
using namespace std;
int n,m;
            int mini=100000;

int arr[4001][2];
int combi[10000][3];
int arr2[10000];
bool found = false;
int ind = 0;
void solve(int i,int d,int f,int s,int t)
{
    if (d >= 3)
    {
        combi[ind][0] = f;
        combi[ind][1] = s;
        combi[ind][2] = t;
        ind++;
        found = true;
        return;
    }
    for (int x = 0;x < m;x++)
    {
        if (arr[x][0] == i){
            if (d == 0)
                f = arr[x][0];
            if (d == 1)
                s = arr[x][0];
            if (d == 2)
                t = arr[x][0];
            solve(arr[x][1],d+1,f,s,t);

        }
    }
}
int main ()
{
    cin >> n >> m;
    int i,j;
    for (i = 0;i < m;i++)
        cin >> arr[i][0] >> arr[i][1];

    for (i = 0;i < m;i++)
    {
        arr2[arr[i][0]]++;
    }
    for (i = 0;i < m;i++)
        solve(arr[i][0],0,0,0,0);

    if (found == false)
    {
        cout << "-1";
    }
    else{
        for (i = 0;i < ind;i++)
            {
                int sum = 0;
                cout << combi[i][0] << " " << combi[i][1] << " " << combi[i][2] << " ";
                cout << "sum of this combi = " << arr2[combi[i][0]] + arr2[combi[i][1]] + arr2[combi[i][2]] << endl;
                cout << "rep : " << arr2[combi[i][0]] << " rep : " << arr2[combi[i][1]] << " rep : " <<arr2[combi[i][2]] << endl;
                mini = min(mini,arr2[combi[i][0]] + arr2[combi[i][1]] + arr2[combi[i][2]]);
            }
    }
     cout << mini;
}