#include <bits/stdc++.h>

using namespace std;

typedef pair<int, int> pii;
typedef long long ll;
const int N = 2000086, MOD = 998244353, INF = 0x3f3f3f3f;
ll res;
int n, m, cnt, w[N];


int l[10], r[10];
int main() {
    cin >> n;
    for (int i = 1; i < n + 1; i++) cin >> l[i] >> r[i], w[i] = i;
    do {
        int t = 0;
        for (int i = 1; i < n + 1; i++) {
            if (r[w[i]] < t) goto NE;
            t = max(t, l[w[i]]);
        }
        res++;
        NE:;
    } while (next_permutation(w + 1, w + n + 1));
    printf("%lld\n", res);
    return 0;
}