#include <bits/stdc++.h>
//#pragma GCC optimize ("03")
#define FastIO ios_base::sync_with_stdio(false) , cin.tie(0) , cout.tie(0)
#define FILES freopen("obiective.in" , "r" , stdin) , freopen("obiective.out" , "w" , stdout)
#define ll long long
#define ull unsigned long long
#define ld long double
#define eb emplace_back
#define pb push_back
#define qwerty1 first
#define qwerty2 second
#define qwerty3 -> first
#define qwerty4 -> second
#define umap unordered_map
#define uset unordered_set
#define pii pair < ll , ll >
#define pq priority_queue
#define dbg(x) cerr << #x << ": " << x << '\n'

namespace FastRead
{
    char __buff[5000];ll __lg = 0 , __p = 0;
    char nc()
    {
        if(__lg == __p){__lg = fread(__buff , 1 , 5000 , stdin);__p = 0;if(!__lg) return EOF;}
        return __buff[__p++];
    }
    template<class T>void read(T&__x)
    {
        T __sgn = 1; char __c;while(!isdigit(__c = nc()))if(__c == '-')__sgn = -1;
        __x = __c - '0';while(isdigit(__c = nc()))__x = __x * 10 + __c - '0';__x *= __sgn;
    }
}

using namespace FastRead;
using namespace std;

const ll N = 1e5 + 10;
const ll M = 1e9 + 7;
const ld PI = acos(-1);
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

int n , m;
stack < int > st;
vector < int > G[N] , g[N] , ctc[N];
int mark[N] , in[N] , out[N] , gr[N];

int cnt = 0;

void dfs(int node)
{
    mark[node] = 1;

    for(auto to : G[node])
        if(!mark[to])
            dfs(to);

    st.push(node);
}

void dfs2(int node)
{
    ctc[cnt].pb(node);
    gr[node] = cnt;
    mark[node] = 2;

    for(auto to : g[node])
        if(mark[to] == 1)
            dfs2(to);
}

signed main()
{
	#ifndef ONLINE_JUDGE
		FastIO , FILES;
	#endif

    int i;

    cin >> n >> m;

    while(m--)
    {
        int x , y;

        cin >> x >> y;
        G[x].pb(y);
        g[y].pb(x);
    }

    for(i = 1 ; i <= n ; i++)
        if(!mark[i])
            dfs(i);

    while(!st.empty())
    {
        int node = st.top();
        st.pop();

        if(mark[node] == 1)
        {
            ++cnt;
            dfs2(node);
        }
    }

    for(i = 1 ; i <= n ; i++)
        for(auto j : G[i])
            if(gr[i] != gr[j])
            {
                ++in[gr[j]];
                ++out[gr[i]];
            }

    int nr = 0;

    for(i = 1 ; i <= cnt ; i++)
        nr += in[i] == 0;

    assert(nr > 1);

    return 0;
}