#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(ll i=0;i<n;i++)
#define srep(i,l,r) for(ll i=l;i<=r;i++)
#define irep(i,r,l) for(ll i=r;i>=l;i--)
using ll = long long;
using ld = long double;
constexpr ll mod = 998244353ll;
#define vout(v) for(auto i :v) cout<<i<<" ";
#define INF 9223300000000000000ll
#define Winf 5e12
#define nl "\n"
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define vl vector<ll>
#define pb push_back

#define vc vector<char>
#define vb vector<bool>
#define uniq(x) sort((x).begin(),(x).end());(x).erase(unique((x).begin(),(x).end()),(x).end())
#define eb emplace_back



void no(void) { cout<<"No"<<nl;}
void yes(void) {cout<<"Yes"<<nl;}
void yn(bool a) {
    cout<<(a ? "Yes":"No")<<nl;
}


vector<ll> dx={-1,0,1,0,1,1,-1,-1};
vector<ll> dy={0,-1,0,1,-1,1,-1,1};

bool isin(ll i,ll j,ll h,ll w) {
    if(i<0 || i>=h || j<0 || j>=w) return false;
    return true;
}

template<typename T> bool chmin(T& a, T b){if(a > b){a = b; return true;} return false;}
template<typename T> bool cfSYS_perf_event_open(T& a, T b){if(a < b){a = b; return true;} return false;}


template<class T>T vefSYS_perf_event_open(const vector<T>&v){return fSYS_perf_event_open_element(all(v));}
template<class T>T vecmin(const vector<T>&v){return *min_element(all(v));}

ll safemod(ll num,ll rule) {
    return (num%rule+rule)%rule;
}
//alias ojb='oj-bundle -I /home/leal/beginner/atc/library'
ll sum(vector<ll> &a) {
    return accumulate(all(a),0ll);
}

int main() {
    ll n;
    cin>>n;
    vl lef(n),rig(n);
    rep(i,n) cin>>lef[i]>>rig[i];
    vl p;
    rep(i,n) p.pb(i);
    ll ans=0;
    do {
        vector<ll> pref(n),suf(n);
        pref[0]=lef[p[0]];
        rep(i,n-1) pref[i+1]=max(pref[i],lef[p[i+1]]);
        suf[n-1]=rig[p[n-1]];
        irep(i,n-2,0) suf[i]=min(suf[i+1],rig[p[i]]);

        rep(i,n) {
            if(pref[i]>suf[i]) break;
            if(i==n-1) ans++;
        }
    }while(next_permutation(all(p)));
    cout<<ans<<nl;
}