# include <bits/stdc++.h>
# include <atcoder/modint>
# include <atcoder/segtree>
# include <atcoder/lazysegtree>
# include <atcoder/dsu>
# include <atcoder/scc>
# include <atcoder/string>
# include <atcoder/twosat>
# include <atcoder/math>
# include <atcoder/convolution>
//# include <regex>

using namespace std;
typedef long long ll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<vector<vector<int>>> vvvi;
typedef vector<long long> vl;
typedef vector<vector<long long>> vvl;
typedef vector<vector<vector<long long>>> vvvl;
typedef vector<bool> vb;
typedef vector<vector<bool>> vvb;
typedef vector<vector<vector<bool>>> vvvb;
#define rep(i,n) for(int i=0;i<n;i++)
#define reps(i,m,n) for(int i=m;i<n;i++)
#define repl(i,n) for(ll i=0;i<n;i++)
#define repsl(i,m,n) for(ll i=m;i<n;i++)
#define repr(i,n) for(int i=n-1;i>=0;i--)
#define repsr(i,m,n) for(int i=n-1;i>=m;i--)
#define replr(i,n) for(ll i=n-1;i>=0;i--)
#define repslr(i,m,n) for(ll i=n-1;i>=m;i--)
#define sksort(x) sort(x.begin(), x.end())
#define sksortr(x) sort(x.rbegin(), x.rend())
#define disp(x) cout << x << endl
#define disps(x) cout << x << " "
#define dispe cout << endl
#define dispv(x) for(ll xqzj=0;xqzj<(ll)x.size();xqzj++){disps(x[xqzj]);}dispe
#define dispvv(x) for(ll xqzi=0;xqzi<(ll)x.size();xqzi++){dispv(x[xqzi]);}
#define dispvm(x) for(ll xqzj=0;xqzj<(ll)x.size();xqzj++){disps(x[xqzj].val());}dispe
#define dispvvm(x) for(ll xqzi=0;xqzi<(ll)x.size();xqzi++){dispvm(x[xqzi]);}
#define dispy cout << "Yes" << endl
#define dispn cout << "No" << endl
#define dispyn(x) if(x)dispy;else dispn
#define dispd cout << std::setprecision(20)
#define inp(x) int x;cin>>x
#define inpl(x) ll x;cin>>x
#define inps(x) string x;cin>>x
#define allv(x)  (x).begin(),(x).end()
#define allrv(x)  (x).rbegin(),(x).rend()
#define imax(x,y) x=max(x,y)
#define imin(x,y) x=min(x,y)
template <class T>
using priority_queue_asc = std::priority_queue<T,std::vector<T>,std::greater<T>>;
using mint = atcoder::modint998244353;
//using mint = atcoder::modint1000000007;

int main(){
    inp(n);
    vi p(n);
    rep(i,n)p[i]=i;
    vi l(n),r(n);
    rep(i,n)cin>>l[i]>>r[i];
    int ans=0;
    do{
        int x=0;
        bool f=true;
        rep(i,n){
            if(r[p[i]]<x){
                f=false;
                break;
            }else imax(x,l[p[i]]);
        }
        if(f)ans++;
    }while(next_permutation(allv(p)));
    disp(ans);
}