import sys
#input = sys.stdin.readline
#input = sys.stdin.buffer.readline #文字列はダメ
#sys.setrecursionlimit(1000000)
#import bisect
#import itertools
#import random
#from heapq import heapify, heappop, heappush
#from collections import defaultdict 
#from collections import deque
#import copy #DeepCopy: hoge = [_[:] for _ in hogehoge]
#import math
#from functools import lru_cache
#@lru_cache(maxsize=None)
#MOD = pow(10,9) + 7
MOD = 998244353
#dx = [1,0,-1,0]
#dy = [0,1,0,-1]
#dx8 = [1,1,0,-1,-1,-1,0,1]
#dy8 = [0,1,1,1,0,-1,-1,-1]

def main():
    H,W = map(int,input().split())
    S = [input() for _ in range(H)]

    ans = [S[0][0]]
    stack = []
    stack.append((S[0][0],0))
    cnt = 0
    while cnt < H+W-2:
        mn = 'zz'
        visited = set([])
        # print(stack)
        while stack:
            v,idx = stack.pop()
            x = idx//W
            y = idx%W
            for i in range(2):
                if i == 0:
                    if x+1 >= H: continue
                    nv = S[x+1][y]
                    nidx = (x+1)*W + y
                else:
                    if y+1 >= W: continue
                    nv = S[x][y+1]
                    nidx = x*W + y+1
                if nidx in visited:
                    continue
                if nv < mn:
                    mn = nv
                    ok = [(nv,nidx)]
                    visited.add(nidx)
                elif nv == mn:
                    ok.append((nv,nidx))
                    visited.add(nidx)
        cnt += 1
        ans.append(mn)
        stack = ok
        # print(stack)
    print("".join(ans))

                

if __name__ == '__main__':
    main()