#include <bits/stdc++.h>
using namespace std;
void dfs(int x, vector<vector<int>> &AR, vector<int> &ordre, vector<int> &occ)
{
occ[x] = 1;
for (int i = 0; i < AR[x].size(); i++)
{
if (occ[AR[x][i]])
continue;
dfs(AR[x][i], AR, ordre, occ);
}
ordre.push_back(x);
}
void dfs2(int x, vector<vector<int>> &AR, vector<int> &occ, vector<int> &comp, vector<int> &color, vector<vector<int>> &comps, int c)
{
color[x] = c;
occ[x] = 1;
comp[x] = comps.size() - 1;
comps.back().push_back(x);
for (int i = 0; i < AR[x].size(); i++)
{
if (occ[AR[x][i]])
continue;
dfs2(AR[x][i], AR, occ, comp, color, comps, 0 - c);
}
}
int main()
{
cin.tie(0);
cout.tie(0);
ios::sync_with_stdio(0);
int N, M;
cin >> N >> M;
vector<vector<int>> AR(N), AR2(N);
vector<int> left(N);
for (int i = 0; i < M; i++)
{
int a, b;
cin >> a >> b;
a--, b--;
AR[a].push_back(b);
AR2[b].push_back(a);
}
vector<int> ordre, occ(N);
for (int i = 0; i < N; i++)
{
if (occ[i])
continue;
dfs(i, AR, ordre, occ);
}
occ.assign(N, 0);
vector<int> comp(N), color(N);
vector<vector<int>> comps;
for (int i = N - 1; i >= 0; i--)
{
if (occ[i])
continue;
comps.push_back({});
dfs2(i, AR2, occ, comp, color, comps, 1);
}
if (comps.size() != N)
{
cout << -1;
return 0;
}
vector<int> dep(comps.size());
vector<vector<int>> AR3(comps.size());
for (int i = 0; i < N; i++)
{
for (int j = 0; j < AR[i].size(); j++)
{
if (comp[i] != comp[AR[i][j]])
AR3[comp[AR[i][j]]].push_back(comp[i]);
else
left[i]++;
}
}
queue<int> Q;
for (int i = 0; i < comps.size(); i++)
{
sort(AR3[i].begin(), AR3[i].end());
vector<int> temp;
for (int j = 0; j < AR3[i].size(); j++)
if (j == 0 || AR3[i][j] != AR3[i][j - 1])
{
temp.push_back(AR3[i][j]);
dep[AR3[i][j]]++;
}
swap(temp, AR3[i]);
if (dep[i] == 0)
Q.push(i);
}
vector<int> ans(N);
while (!Q.empty())
{
int x = Q.front();
Q.pop();
queue<pair<int, int>> Q2;
for (int i = 0; i < comps[x].size(); i++)
{
int y = comps[x][i];
for (int j = 0; j < AR[y].size(); j++)
{
if (ans[AR[y][j]] == 1)
Q2.push({y, 0});
}
}
while (!Q2.empty())
{
int y = Q2.front().first, t = Q2.front().second;
Q2.pop();
if (t)
{
ans[y] = 1;
for (int i = 0; i < AR2[y].size(); i++)
{
if (comp[AR2[y][i]] != x)
continue;
if (left[AR2[y][i]])
Q2.push({AR2[y][i], 0});
left[AR2[y][i]] = 0;
}
}
else
{
ans[y] = -1;
for (int i = 0; i < AR2[y].size(); i++)
{
if (comp[AR2[y][i]] != x)
continue;
left[AR2[y][i]]--;
if (left[AR2[y][i]] == 0)
Q2.push({AR2[y][i], 1});
}
}
}
for (int i = 0; i < comps[x].size(); i++)
{
if (ans[comps[x][i]] == 0)
ans[comps[x][i]] = color[comps[x][i]];
}
for (int i = 0; i < AR3[x].size(); i++)
{
dep[AR3[x][i]]--;
if (dep[AR3[x][i]] == 0)
Q.push(AR3[x][i]);
}
}
int tot = 0;
for (int i = 0; i < N; i++)
if (ans[i] == 1)
tot++;
cout << tot << '\n';
for (int i = 0; i < N; i++)
if (ans[i] == 1)
cout << i + 1 << ' ';
}
