#include <bits/stdc++.h>

#include <atcoder/all>
using namespace std;
using i32 = int;
using u32 = unsigned int;
using i64 = long long;
using u64 = unsigned long long;
#define FAST_IO                \
  ios::sync_with_stdio(false); \
  cin.tie(0);
const i64 INF = 1001001001001001001;
using Modint = atcoder::static_modint<998244353>;

int main() {
  FAST_IO

  int N;
  cin >> N;
  vector<int> L(N), R(N);
  for (int i = 0; i < N; i++) {
    cin >> L[i] >> R[i];
  }

  int ans = 0;
  vector<int> idx(N);
  iota(idx.begin(), idx.end(), 0);

  do {
    bool valid = true;
    int cur = 0;
    for (int i = 0; i < N; i++) {
      cur = max(cur, L[idx[i]]);
      if (cur > R[idx[i]]) {
        valid = false;
        break;
      }
    }
    if (valid) {
      ans ++;
    }
  } while (next_permutation(idx.begin(), idx.end()));

  cout << ans << endl;
}