489D - Unbearable Controversy of Being

| In | 
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#include <iostream>
#include <vector>
using namespace std;
int n, m, a, b, rhombi = 0;
int damm[3001];
bool isVisit[3001];
vector<int> road[3001];
inline void DFS(int i, int step) {
    int r;
    for (int j = 0; j < road[i].size(); ++j) {
        r = road[i][j];
        if (!isVisit[r]) {
            isVisit[r] = true;
            if (step < 2) { DFS(r, step + 1); }
            else { ++damm[r]; }
            isVisit[r] = false;
        }
    }
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n >> m;
    for (int i = 0; i < m; ++i) {
        cin >> a >> b;
        road[a].push_back(b);
    }
    for (int i = 1; i <= n; ++i) {
        isVisit[i] = true;			
        DFS(i, 1);
        for (int i = 1; i <= n; ++i) {
            if (damm[i] >= 2) {
                rhombi += damm[i] * (damm[i] - 1) / 2;
            }
            damm[i] = 0;
        }
        isVisit[i] = false;
    }
    cout << rhombi << '\n';
    return 0;
}