use proconio::input;

fn main() {
    input! {
        n: usize,
        lr: [(i64, i64); n],
    }

    println!("{}", dfs(0, n, &mut vec![false; n], &mut vec![0; n], &lr));
}

fn dfs(i: usize, n: usize, used: &mut [bool], permutation: &mut [usize], lr: &[(i64, i64)]) -> i64 {
    if i >= n {
        return if is_valid(permutation, lr) { 1 } else { 0 };
    }
    let mut res = 0;
    for v in 0..n {
        if used[v] {
            continue;
        }
        used[v] = true;
        permutation[i] = v;
        res += dfs(i + 1, n, used, permutation, lr);
        used[v] = false;
    }
    res
}

fn is_valid(permutation: &[usize], lr: &[(i64, i64)]) -> bool {
    let mut difficulty = 0;
    for (l, r) in permutation.iter().map(|&i| lr[i]) {
        if difficulty > r {
            return false;
        }
        difficulty = difficulty.max(l);
    }
    true
}