use proconio::input;

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

    let mut index: Vec<usize> = (0..n).collect();
    let mut ans = 0;
    loop {
        let mut diff = 0;
        let mut flag = true;

        for &i in &index {
            let (l, r) = lr[i];
            if diff > r {
                flag = false;
            }

            diff = diff.max(l);
        }

        if flag {
            ans += 1;
        }

        if !next_permutation(&mut index) {
            break;
        }
    }
    println!("{}", ans);
}

fn next_permutation(arr: &mut Vec<usize>) -> bool {
    let mut l = usize::MAX;
    for i in (0..arr.len() - 1).rev() {
        if arr[i] < arr[i + 1] {
            l = i;
            break;
        }
    }

    if l == usize::MAX {
        return false;
    }

    let mut r = usize::MAX;
    for i in (0..arr.len()).rev() {
        if arr[l] < arr[i] {
            r = i;
            break;
        }
    }

    arr.swap(l, r);
    arr[l + 1..].reverse();
    return true;
}