use proconio::input;

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

    let rest = (1 << n) - 1;
    let mut perm = vec![];
    enumerate(n, &mut perm, &mut vec![], rest);

    let ans = perm
        .iter()
        .filter(|v| {
            let mut ok = true;
            for j in 0..n {
                for i in 0..j {
                    if range[v[i]].0 > range[v[j]].1 {
                        ok = false;
                    }
                }
            }
            ok
        })
        .count();
    println!("{ans}");
}

fn enumerate(n: usize, res: &mut Vec<Vec<usize>>, state: &mut Vec<usize>, rest: usize) {
    if rest == 0 {
        res.push(state.clone());
        return;
    }

    for i in 0..n {
        if rest >> i & 1 == 1 {
            state.push(i);
            enumerate(n, res, state, rest ^ 1 << i);
            state.pop();
        }
    }
}