import sys
readline=sys.stdin.readline

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)
    prev=queue
    queue=[]
    for h,w in prev:
        if h+1<H and S[h+1][w]==s:
            queue.append((h+1,w))
        if w+1<W and S[h][w+1]==s:
            if queue and (h,w+1)==queue[-1]:
                continue
            queue.append((h,w+1))
print(*ans_lst,sep="")