Конденсация

#pragma GCC optimize("O3")
#pragma GCC target("avx2")
#include <bits/stdc++.h>
#include <ext/rope>
#include <ext/pb_ds/assoc_container.hpp>
#define endl '\n'
#define fixstd() ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0)
#define precision(n) cout.setf(ios::fixed), cout.precision(n)
#define all(arr) arr.begin(), arr.end()
#define rall(arr) arr.rbegin(), arr.rend()
#define sz(arr) int(arr.size())
#define sqr(n) ((n) * (n))
#define print(arr) {for (auto el: arr) cout << el << ' '; cline();}
#define pprint(A) for (auto &x: A) print(x)
#define cprint(A) {for (auto &x: A) cout << x.fi << ',' << x.se << ' '; cline();}
#define read(arr) for (auto &el: arr) cin >> el
#define make_uniq(arr) sort(all(arr)), arr.resize(unique(all(arr)) - arr.begin());
#define inc(arr) for (auto &x: arr) ++x
#define dec(arr) for (auto &x: arr) --x
#define jumpout() assert(false)
#define multitest() int _; cin >> _; for (; _; --_)
#define yesno(fl) (fl ? "YES" : "NO")
#define out(n) cout << n << endl, exit(0);
#define write(n) cout << n << endl
#define debug(s, n) cout << s << ": " << n << endl
#define cline() cout << endl
#define pb push_back
#define mp make_pair
#define fi first
#define se second
using namespace std;
using namespace __gnu_cxx;
using namespace __gnu_pbds;
typedef tree<int, null_type, less<int>, rb_tree_tag,
tree_order_statistics_node_update> indexed_set;
typedef long double ld;
typedef string str;
#define int int64_t
typedef pair<int, int> col;
typedef pair<ld, ld> cold;
typedef pair<str, str> cols;

int n, m, num_comp;
vector<vector<int>> gr, rgr;
vector<bool> used;
vector<int> tops, comp;
vector<set<int>> cgr;

void init() {
    num_comp = 0;
    gr.resize(n);
    rgr.resize(n);
    comp.assign(n, -1);
}

void dfs(int v) {
    if (used[v]) return;
    used[v] = true;
    for (int u: gr[v])
        dfs(u);
    tops.pb(v);
}

void rdfs(int v) {
    if (used[v]) return;
    used[v] = true;
    comp[v] = num_comp;
    for (int u: rgr[v])
        rdfs(u);
}

int solve() {
    used.assign(n, false);
    for (int v = 0; v < n; ++v)
        dfs(v);
    reverse(all(tops));
    used.assign(n, false);
    for (int v: tops) {
        if (used[v]) continue;
        rdfs(v);
        ++num_comp;
    }
    cgr.resize(num_comp);
    for (int v = 0; v < n; ++v) {
        for (int u: gr[v]) {
            int cv = comp[v], cu = comp[u];
            if (cv == cu) continue;
            cgr[cv].insert(cu);
        }
    }
    int ans = 0;
    for (int v = 0; v < num_comp; ++v)
        ans += sz(cgr[v]);
    return ans;
}

signed main() {
    fixstd();
    cin >> n >> m, init();
    for (int i = 0; i < m; ++i) {
        int v, u; cin >> v >> u, --v, --u;
        gr[v].pb(u), rgr[u].pb(v);
    }
    int ans = solve();
    write(ans);
}