use std::collections::HashSet;

fn main() {
    let mut hw = String::new();
    std::io::stdin().read_line(&mut hw).ok();
    let hw: Vec<usize> = hw.trim().split_whitespace().map(|s| s.parse().unwrap()).collect();
    let h = hw[0];
    let w = hw[1];
    let grid = (0..h).map(|_| {
            let mut temp = String::new();
            std::io::stdin().read_line(&mut temp).ok();
            let temp = temp.trim();
            temp.chars().collect::<Vec<char>>()
        })
        .collect::<Vec<Vec<char>>>();

    let mut result = vec![];
    let mut fronts = HashSet::new();
    result.push(grid[0][0]);
    fronts.insert((0, 0));
    for _ in 0..h+w-2 {
        let mut nexts = HashSet::new();
        let mut candc = None;
        for &(x, y) in fronts.iter() {
            if x + 1 < h {
                if candc.is_none() {
                    candc = Some(grid[x+1][y]);
                    nexts.insert((x+1, y));
                } else if candc.unwrap() as usize > grid[x+1][y] as usize {
                    candc = Some(grid[x+1][y]);
                    nexts = HashSet::new();
                    nexts.insert((x+1, y));
                } else if candc.unwrap() as usize == grid[x+1][y] as usize {
                    nexts.insert((x+1, y));
                }
            }
            if y + 1 < w {
                if candc.is_none() {
                    candc = Some(grid[x][y+1]);
                    nexts.insert((x, y+1));
                } else if candc.unwrap() as usize > grid[x][y+1] as usize {
                    candc = Some(grid[x][y+1]);
                    nexts = HashSet::new();
                    nexts.insert((x, y+1));
                } else if candc.unwrap() as usize == grid[x][y+1] as usize {
                    nexts.insert((x, y+1));
                }
            }
        }
        result.push(candc.unwrap());
        fronts = nexts;
    }
    for c in result.into_iter() { print!("{}", c); }
    println!();
}