#![allow(unused_imports, unused_macros, non_snake_case)]
use std::cmp::{max, min, Ordering};
use std::collections::{BTreeMap, BTreeSet, BinaryHeap, HashMap, HashSet, VecDeque};

//{{{ macros missing in 1.15.1
#[macro_use]
mod macros {
    macro_rules! eprint {
        ($($t:tt)*) => {{
            use ::std::io::Write;
            let _ = write!(::std::io::stderr(), $($t)*);
        }};
    }
    macro_rules! eprintln {
        () => { eprintln!(""); };
        ($($t:tt)*) => {{
            use ::std::io::Write;
            let _ = writeln!(::std::io::stderr(), $($t)*);
        }};
    }
    macro_rules! dbg {
        ($val:expr) => {
            match $val {
                tmp => {
                    eprintln!(
                        "[{}:{}] {} = {:#?}",
                        file!(),
                        line!(),
                        stringify!($val),
                        &tmp
                    );
                    tmp
                }
            }
        };
    }
}
//}}}

// (init, count)
fn solve(init: usize, d: usize, c: usize) -> (usize, usize) {
    let mut dp = HashMap::new();
    // (iter, cost)
    dp.insert(init, (0, 0));
    let mut cur = init;
    let mut cost = 0;
    let mut iter = 0;
    while iter < c {
        cur = cur + d;
        cost += 1;
        if cur >= 10 {
            cur = cur % 10 + 1;
            cost += 1;
        }
        iter += 1;
        if dp.contains_key(&cur) {
            let diff = iter - dp[&cur].0;
            let diff_c = cost - dp[&cur].1;
            let times = (c - iter) / diff;
            iter += times * diff;
            cost += times * diff_c;
        } else {
            dp.insert(cur, (iter, cost));
        }
    }
    (cur, cost)
}

fn run() {
    let stdin = std::io::stdin();
    let mut sc = Scanner::new(stdin.lock());
    let m: usize = sc.read();
    let mut dc: Vec<(usize, usize)> = Vec::new();
    let mut cur = 0;
    let mut answer = 0;
    for _ in 0..m {
        let d: usize = sc.read();
        let c: usize = sc.read();
        dc.push((d, c));
        let (next, add) = solve(cur, d, c);
        cur = next;
        answer += add;
    }
    println!("{}", answer - 1);
}

fn main() {
    std::thread::Builder::new()
        .name("run".to_string())
        .stack_size(1024 * 1024 * 1024)
        .spawn(run)
        .unwrap()
        .join()
        .unwrap()
}

//{{{ Scanner
pub struct Scanner<R> {
    reader: R,
}

impl<R> Scanner<R> {
    pub fn new(r: R) -> Scanner<R> {
        Scanner { reader: r }
    }
}

fn is_whitespace(b: u8) -> bool {
    b == b' ' || b == b'\n' || b == b'\r' || b == b'\t'
}

impl<R: std::io::Read> Scanner<R> {
    pub fn read<T: std::str::FromStr>(&mut self) -> T {
        let buf = std::io::Read::bytes(self.reader.by_ref())
            .map(|b| b.expect("Read failed"))
            .skip_while(|&b| is_whitespace(b))
            .take_while(|&b| !is_whitespace(b))
            .collect::<Vec<_>>();
        unsafe { std::str::from_utf8_unchecked(&buf) }
            .parse()
            .ok()
            .expect("Parse error")
    }

    pub fn read_vec<T: std::str::FromStr>(&mut self, n: usize) -> Vec<T> {
        (0..n).map(|_| self.read()).collect()
    }

    pub fn read_chars(&mut self) -> Vec<char> {
        self.read::<String>().chars().collect()
    }
}
//}}}


Measured memory_bytes: 13536