#include<iostream>
#include<cstring>
#include<cstdio>
#include<vector>
#include<queue>

using namespace std;
const int N = 10000 + 15;
int n, m, in[N], val[N];
vector<int> edge[N];

void Solve_question(){
    int ans = 0, cnt = 0;
    queue <int> Q;

    for(int i = 1; i <= n; i++)
        if(!in[i]) { Q.push(i); val[i] = 888; }
    while(!Q.empty()){
        int u = Q.front(); Q.pop();
        cnt++;
        for(int i = 0; i < (int)edge[u].size(); i++){
            int v = edge[u][i];
            if(--in[v] == 0){
                Q.push(v);
                val[v] = val[u] + 1;
            }
        }
    }
    if(cnt < n) puts("-1");
    else{
        for(int i = 1; i <= n; i++)
            ans += val[i];
        printf("%d\n", ans);
    }
}

void Input_data(){
    for(int i = 1; i <= n; i++) edge[i].clear(), val[i] = in[i] = 0;
    int u, v;
    for(int i = 1; i <= m; i++){
        scanf("%d %d", &u, &v);
        in[u]++;
        edge[v].push_back(u);
    }
}

int main(){
    while(scanf("%d %d", &n, &m) == 2){
        Input_data();
        Solve_question();
    }
}