macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        let mut next = || { iter.next().unwrap() };
        input_inner!{next, $($r)*}
    };
    ($($r:tt)*) => {
        let stdin = std::io::stdin();
        let mut bytes = std::io::Read::bytes(std::io::BufReader::new(stdin.lock()));
        let mut next = move || -> String{
            bytes
                .by_ref()
                .map(|r|r.unwrap() as char)
                .skip_while(|c|c.is_whitespace())
                .take_while(|c|!c.is_whitespace())
                .collect()
        };
        input_inner!{next, $($r)*}
    };
}

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

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

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

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

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

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

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

fn main() {
    input! {
        n: usize,
        m: i64,
        scs: [[i64; 2]; m],
    };

    let mut guess = vec![-1; n];
    let mut flag = true;

    for sc in scs {
        let s = (sc[0] - 1) as usize;
        let c = sc[1];

        if guess[s] == -1 {
            guess[s] = c
        }
        if guess[s] != c || (guess[0] == 0 && guess.len() > 1) {
            flag = false
        }

        if !flag {
            break;
        }
    }

    if !flag {
        println!("-1");
    } else {
        guess[0] = if guess.len() == 1 && guess[0] == -1 {
            0
        } else if guess[0] == -1 {
            0
        } else {
            guess[0]
        };
        for i in 1..guess.len() {
            guess[i] = std::cmp::max(guess[i], 0);
        }

        for c in guess {
            print!("{}", c);
        }
        println!("");
    }
}
