macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        #[allow(unused_mut)]
        let mut s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};

    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };

    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };

    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };

    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };

    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}


use std::cmp;

fn main() {
    input![
        n: u32,
        m: u32,
        s: [[u32; 2]; m]
    ];

    let mut v: Vec<u32> = (0..n).map(|_| 0).collect();
    for item in s{
        let digit = (item[0] -1) as usize;
        let num  = item[1];
        v[digit] = cmp::min(num, v[digit]);
    }
    let result: i32 = if v[0] == 0 {
        -1
    } else {
        let mut r = 0;
        let len = v.len();
        for i in 0..(len){
            r += 10u32.pow((len - i -1) as u32) * v[i];
        }
        r as i32
    };

    println!("{}", result);
}
