P4645 [COCI 2006/2007 #3] BICIKLI

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N = 1e6 + 10, mod = 1e9;
int n, m, in[N], dp[N], u[N], v[N];
bool vis[N], topo[N];
vector<int> G[N];
void dfs(int u){
	vis[u] = 1;
	for(int v : G[u]){
		if(!vis[v]) dfs(v);
	}
}
signed main() {
	ios::sync_with_stdio(false);
	cin.tie(0), cout.tie(0);

	cin >> n >> m;
	for(int i = 1, u, v; i <= m; i ++){
		cin >> u >> v;
		G[u].push_back(v);
		in[v] ++;
	}
	dfs(1);
	for(int i = 1; i <= n; i ++){
		if(!vis[i]){
			for(int v : G[i]) in[v] --;
		}
	}
	queue<int> Q;
	Q.push(1), dp[1] = 1;
	while(!Q.empty()){
		int u = Q.front();
		Q.pop();
		for(int v : G[u]){
			if(!vis[v]) continue;
			topo[v] = 1;
			dp[v] = (dp[v] + dp[u]) % mod;
			if((-- in[v]) == 0) Q.push(v);
		}
	}
	for(int i = 1; i <= n; i ++){
		if(topo[i] && in[i]&& vis[i]){
			cout << "inf";
			return 0;
		}
	}
	cout << dp[2];
	return 0;
}