use proconio::input;

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

    let mut count = [0usize; 4];
    for &(a, b) in &cards {
        let i = a.trailing_zeros().min(3) as usize;
        count[i] += b;
    }

    let mut ans = count[3];

    let m = count[1].min(count[2]);
    count[1] -= m;
    count[2] -= m;
    ans += m;

    ans += count[1] / 3;
    ans += count[2] / 2;

    println!("{ans}");
}