import bisect
import copy
import decimal
import fractions
import heapq
import itertools
import math
import random
import sys
import time
from collections import Counter,deque,defaultdict
from functools import lru_cache,reduce
from heapq import heappush,heappop,heapify,heappushpop,_heappop_max,_heapify_max
def _heappush_max(heap,item):
    heap.append(item)
    heapq._siftdown_max(heap, 0, len(heap)-1)
def _heappushpop_max(heap, item):
    if heap and item < heap[0]:
        item, heap[0] = heap[0], item
        heapq._siftup_max(heap, 0)
    return item
from math import gcd as GCD
read=sys.stdin.read
readline=sys.stdin.readline
readlines=sys.stdin.readlines
write=sys.stdout.write

H,W=map(int,readline().split())
S=[readline().rstrip() for h in range(H)]
ans_lst=[S[0][0]]
queue=[(0,0)]
for _ in range(H+W-2):
    s=min([S[h+1][w] for h,w in queue if h+1<H]+[S[h][w+1] for h,w in queue if w+1<W])
    ans_lst.append(s)
    queue=list({(h+1,w) for h,w in queue if h+1<H and S[h+1][w]==s}|{(h,w+1) for h,w in queue if w+1<W and S[h][w+1]==s})
print(*ans_lst,sep="")