import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }




void main() {
  try {
    for (; ; ) {
      const H = readInt;
      const W = readInt;
      auto S = new string[H];
      foreach (x; 0 .. H) {
        S[x] = readToken;
      }
      
      auto ans = new char[H + W - 1];
      auto dp = new bool[][](H, W);
      dp[0][0] = true;
      foreach (s; 0 .. H + W - 1) {
        ans[s] = 'z' + 1;
        foreach (x; 0 .. H) {
          const y = s - x;
          if (0 <= y && y < W && dp[x][y]) {
            chmin(ans[s], S[x][y]);
          }
        }
        foreach (x; 0 .. H) {
          const y = s - x;
          if (0 <= y && y < W && dp[x][y]) {
            dp[x][y] = (ans[s] == S[x][y]);
            if (dp[x][y]) {
              if (x + 1 < H) dp[x + 1][y] = true;
              if (y + 1 < W) dp[x][y + 1] = true;
            }
          }
        }
      }
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}