fn main() {
    input! {
        h: usize,
        w: usize,
        s: [bytes; h],
    }
    let mut ans = vec![];
    let mut pos = vec![(0, 0)];
    for _ in 0..(h + w - 1) {
        let (x, y) = pos[0];
        ans.push(s[x][y]);
        let mut next = vec![];
        for (x, y) in pos {
            if x + 1 < h {
                next.push((s[x + 1][y], x + 1, y));
            }
            if y + 1 < w {
                next.push((s[x][y + 1], x, y + 1));
            }
        }
        next.sort();
        next.dedup();
        if next.len() > 0 {
            let c = next[0].0;
            next.retain(|p| p.0 == c);
        }
        pos = next.into_iter().map(|p| (p.1, p.2)).collect();
    }
    let ans = ans.into_iter().map(|c| c as char).collect::<String>();
    println!("{}", ans);
}

// ---------- begin input macro ----------
// reference: https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
#[macro_export]
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let 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_export]
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_export]
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, bytes) => {
        read_value!($iter, String).bytes().collect::<Vec<u8>>()
    };
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}
// ---------- end input macro ----------