import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int h = sc.nextInt();
        int w = sc.nextInt();
        char[][] field = new char[h][];
        for (int i = 0; i < h; i++) {
            field[i] = sc.next().toCharArray();
        }
        StringBuilder ans = new StringBuilder();
        HashSet<Integer> current = new HashSet<>();
        HashSet<Integer> next = new HashSet<>();
        current.add(0);
        for (int i = 0; i < h + w - 1; i++) {
            char min = (char)('z' + 1);
            for (int x : current) {
                int r = x / w;
                int c = x % w;
                if (ans.length() == i) {
                    ans.append(field[r][c]);
                }
                if (r < h - 1) {
                    if (field[r + 1][c] < min) {
                        min = field[r + 1][c];
                        next.clear();
                        next.add((r + 1) * w + c);
                    } else if (field[r + 1][c] == min) {
                        next.add((r + 1) * w + c);
                    }
                }
                if (c < w - 1) {
                    if (field[r][c + 1] < min) {
                        min = field[r][c + 1];
                        next.clear();
                        next.add(r * w + c + 1);
                    } else if (field[r][c + 1] == min) {
                        next.add(r * w + c + 1);
                    }
                }
            }
            HashSet<Integer> tmp = next;
            next = current;
            current = tmp;
            next.clear();
        }
        System.out.println(ans);
    }
}
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
    
}