#include <bits/stdc++.h>
using namespace std;
const int nmax = 2e5;
int n, m;
int b[nmax + 5], t[nmax + 5];
vector<pair<int,int>> l[nmax + 5];
int sel[nmax + 5];
int nr = 0;
int found = 0;
bool over(int a)
{
if(l[a].front().second != 1 || l[a].back().second != 1)
{
return false;
}
return true;
}
void dfs(int nod, int poz, bool lvl)
{
if(over(nod) && sel[nod] == 0)
{
++found;
}
if(!sel[nod])
{
++nr;
}
++sel[nod];
if(sel[nod] == 1)
{
for(auto it : l[nod])
{
if(poz == it.first && lvl == it.second)
{
continue;
}
dfs(nod, it.first, it.second);
}
}
if(lvl == 0)
{
if(!sel[t[poz]])
{
dfs(t[poz], poz, 1);
}
}
else
{
if(!sel[b[poz]])
{
dfs(b[poz], poz, 0);
}
}
}
int main()
{
ios::sync_with_stdio(false);
cin.tie(0);
#ifdef home
freopen("nr.in","r",stdin);
freopen("nr.out","w",stdout);
#endif // home
cin>>n>>m;
bool done = true;
for(int i=1; i<=m; i++)
{
cin>>b[i]>>t[i];
l[b[i]].push_back({i, 0});
l[t[i]].push_back({i, 1});
if(t[i] != b[i])
{
done = false;
}
}
if(n == m && !done)
{
cout<<-1<<'\n';
return 0;
}
bool ok = false;
int rez = 0;
for(int i=1; i<=m; i++)
{
nr = 0;
found = 0;
if(t[i] != 0 && !sel[t[i]])
{
dfs(t[i], i, 1);
}
if(nr <= 1)
{
continue;
}
rez += (nr + (found > 1) + 1);
if(found > 1)
{
ok = true;
}
}
if(ok && m == n + 1)
{
cout<<-1<<'\n';
return 0;
}
cout<<rez<<'\n';
return 0;
}
