参考答案：

#include<iostream>
#include<cstring>
using namespace std;
const int N = 25000;
int e[N], ne[N], h[N], idx;
int b[N], dis[N];
int q[N];
int n, m;
void add(int a, int b)
{
    e[idx] = b, ne[idx] = h[a], h[a] = idx++;
}
bool top_sort()
{
    int hh = 0, tt = -1;
    for (int i = 1; i <= n; ++i) if (!b[i]) q[++tt] = i;
    while (hh <= tt)
    {
        int t = q[hh++];
        for (int i = h[t]; ~i; i = ne[i])
        {
            int j = e[i];
            if (--b[j] == 0)
            {
                q[++tt] = j;
            }
        }
    }
    return tt == n - 1;
}
int main()
{
    memset(h, -1, sizeof(h));
    cin >> n >> m;
    for (int i = 1; i <= m; ++i)
    {
        int a, c;
        cin >> a >> c;
        add(c, a);
        b[a]++;
    }
    if (!top_sort()) cout << "No answer" << endl;
    else
    {
        for (int i = 1; i <= n; ++i) dis[i] = 100;
        for (int i = 0; i < n; ++i)
        {
            int j = q[i];
            for (int k = h[j]; ~k; k = ne[k])
            {
                dis[e[k]] = max(dis[e[k]], dis[j] + 1);
            }
        }
        int res = 0;
        for (int i = 1; i <= n; ++i) res += dis[i];
        cout << res << endl;
    }
    return 0;
}