import java.io.*;
import java.util.Arrays;

public class Main {
    static PrintWriter pw = new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
    static StreamTokenizer st = new StreamTokenizer(new BufferedReader(new InputStreamReader(System.in)));

    static final int N = 10010, M = 50010;
    static int[] h = new int[N], e = new int[M], ne = new int[M];
    static int[] dfn = new int[N], low = new int[N];
    static int[] stk = new int[N], id = new int[N];
    static int[] size = new int[N]; // 表示每个SCC中点的数量
    static int[] out = new int[N]; // 记录每个SCC的出度
    static boolean[] in_stk = new boolean[N];
    static int n, t, idx, time, scc_cnt, top;
    public static void main(String[] args) throws Exception {
        Arrays.fill(h, -1);

        n = getInt(); t = getInt();

        while (t-- > 0) {
            int a = getInt(), b = getInt();
            add(a, b);
        }

        for (int i = 1; i <= n; i++) {
            if (dfn[i] == 0) {
                tarjan(i);
            }
        }

        for (int i = 1; i <= n; i++) {
            for (int j = h[i]; j != -1; j = ne[j]) {
                int k = e[j];
                int a = id[i], b = id[k];
                if (a != b) {
                    out[a]++;
                }
            }
        }

        int zeros = 0; // 统计有多少个点的出度为0
        int sum = 0; // 存出度为0的SCC内点的个数之和
        for (int i = 1; i <= scc_cnt; i++) {
            if (out[i] == 0) {
                zeros++;
                sum += size[i];
                if (zeros > 1) {
                    sum = 0;
                    break;
                }
            }
        }
      
        pw.println(sum);
        pw.flush();
    }

    static void tarjan(int u) {
        dfn[u] = low[u] = ++time;
        stk[++top] = u; in_stk[u] = true;

        for (int i = h[u]; i != -1; i = ne[i]) {
            int j = e[i];
            if (dfn[j] == 0) {
                tarjan(j);
                low[u] = Math.min(low[u], low[j]);
            } else if (in_stk[j]) {
                low[u] = Math.min(low[u], dfn[j]);
            }
        }

        if (dfn[u] == low[u]) {
            int y;
            ++scc_cnt;
            do {
                y = stk[top--];
                in_stk[y] = false;
                id[y] = scc_cnt;
                size[scc_cnt]++;
            } while (y != u);
        }
    }

    static void add(int a, int b) {
        e[idx] = b;
        ne[idx] = h[a];
        h[a] = idx++;
    }

    static int getInt() throws Exception {
        st.nextToken();
        return (int) st.nval;
    }
}