use amplitude::Amplitude;

fn main() {
    let n = {
        let mut line = String::new();
        std::io::stdin().read_line(&mut line).unwrap();
        line.trim().parse::<usize>().unwrap()
    };
    let xy: Vec<_> = (0..n)
        .map(|_| {
            let mut line = String::new();
            std::io::stdin().read_line(&mut line).unwrap();
            let mut iter = line.split_whitespace();
            (
                iter.next().unwrap().parse::<usize>().unwrap(),
                iter.next().unwrap().parse::<usize>().unwrap(),
            )
        })
        .collect();

    let mut amplitudes: Vec<_> = xy
        .iter()
        .enumerate()
        .map(|(i, &(x, y))| (Amplitude::new(x, y), i))
        .collect();
    amplitudes.sort_unstable_by_key(|x| x.0);

    let mut ans = vec![];
    for i in (0..(n - 1)).step_by(2) {
        ans.push((amplitudes[i].1, amplitudes[i + 1].1))
    }

    println!("{}", ans.len());
    for e in ans {
        println!("{} {}", e.0 + 1, e.1 + 1);
    }
}

pub mod amplitude {
    #[derive(Debug, Clone, Copy, PartialEq, Eq)]
    pub struct Amplitude {
        pub x: usize,
        pub y: usize,
    }

    impl PartialOrd for Amplitude {
        fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
            if self.x == other.x {
                self.y.partial_cmp(&other.y)
            } else {
                self.x.partial_cmp(&other.x)
            }
        }
    }

    impl Ord for Amplitude {
        fn cmp(&self, other: &Self) -> std::cmp::Ordering {
            self.partial_cmp(other).unwrap()
        }
    }

    impl Amplitude {
        pub fn new(x: usize, y: usize) -> Self {
            Self { x, y }
        }
    }
}