This submission is migrated from previous version of oj.uz, which used different machine for grading. This submission may have different result if resubmitted.
#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
#define lg long long
#define fastio ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
const lg N = 2e5+5;
vector<lg> adj[N];
lg in[N], state[N];
int main()
{
fastio;
lg n, m;
cin >> n >> m;
for(int i = 0; i < m; i++)
{
lg u, v;
cin >> u >> v;
adj[u].push_back(v);
in[v]++;
}
queue<lg> q;
lg z = 0;
for(int i = 1; i <= n; i++)
{
if(in[i]) continue;
z++;
q.push(i);
}
cout << n << '\n';
vector<lg> a;
while(q.size())
{
lg u = q.front();
q.pop();
lg x = 0;
for(auto it : adj[u])
{
if(!x) a.push_back(u*2+1);
x++;
in[it]--;
if(!in[it])
{
q.push(it);
z++;
}
}
if(adj[u].empty())
{
a.push_back(u*2);
}
}
if(z < n)
{
cout << "-1\n";
return 0;
}
reverse(a.begin(), a.end());
for(auto it : a) cout << it/2 << ' ' << it%2 << '\n';
return 0;
}
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