#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define rep(i, s, e) for (int i = (int)(s); i < (int)(e); ++i)
#define all(a) (a).begin(),(a).end()

int main() {
    cin.tie(nullptr);
    ios_base::sync_with_stdio(false);
    
    int N;
    cin >> N;
    vector<int> L(N), R(N);
    rep(i, 0, N) cin >> L[i] >> R[i];
    
    vector<int> P(N);
    rep(i, 0, N) P[i] = i;
    
    int ans = 0;
    do {
        int t = 0;
        bool can = true;
        rep(i, 0, N) {
            if (R[P[i]] < t) can = false;
            else t = max(t, L[P[i]]);
        }
        
        if (can) ++ans;
        
    } while (next_permutation(all(P)));
    
    cout << ans << '\n';
}