$N$ oligarchs built a country with $N$ cities with each one of them owning one city. In addition, each oligarch built some roads such that the maximal amount of roads an oligarch can build between two cities is $1$ (note that there can be more than $1$ road going through two cities, but they would belong to different oligarchs).
A total of $d$ roads were built. Some oligarchs wanted to create a corporation by combining their cities and roads so that from any city of the corporation you can go to any city of the corporation using only corporation roads (roads can go to other cities outside corporation) but it turned out that no group of less than $N$ oligarchs can create a corporation. What is the maximal amount that $d$ can have?
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   We need to show that the maximum number of roads \(d\) is \(\binom{N}{3}\). We can achieve this by labeling the oligarchs \(1, 2, \ldots, N\). For \(1 \leq i \leq N\), let the \(i\)-th oligarch build roads between the cities owned by oligarchs \(j\) and \(k\) for all \(1 \leq j < k < i\).

3. **Verification of the Construction:**
   For any set \(S\) of fewer than \(N\) oligarchs, it is impossible to reach the city of the oligarch in \(S\) with the largest label from any city of the other oligarchs in \(S\). This ensures that no group of fewer than \(N\) oligarchs can form a connected corporation.

4. **Optimality of \(\binom{N}{3}\):**
   - For \(N \leq 3\), it can be easily verified that \(\binom{N}{3}\) is optimal.
   - For \(N \geq 4\), we need to show that \(\binom{N}{3}\) is the upper bound.

5. **Counting Good Triples:**
   - A good triple \((a, b, c)\) is one where oligarch \(a\) builds a road connecting the cities of oligarchs \(b\) and \(c\).
   - The number of good triples is precisely twice the number of constructed roads, or \(2d\), because if \((a, b, c)\) is counted, so is \((a, c, b)\).

6. **Distinct Oligarchs in Good Triples:**
   - \(a, b, c\) must be distinct in any good triple.
   - If both \((a, b, c)\) and \((b, a, c)\) are good, the set of two oligarchs \(\{a, b\}\) violates the condition.

7. **Upper Bound on Good Triples:**
   - For each unordered triple \((x, y, z)\) of distinct oligarchs, at most two out of the six permutations are good.
   - Therefore, there are at most \(2 \binom{N}{3}\) good triples.

8. **Conclusion:**
   - Since the number of good triples is \(2d\), we have \(2d \leq 2 \binom{N}{3}\).
   - Thus, \(d \leq \binom{N}{3}\).

The final answer is \(\boxed{\binom{N}{3}}\).