import sys
input = sys.stdin.readline
H,W=map(int,input().split())
S=[list(map(lambda x:ord(x),input())) for _ in range(H)]
INF=1<<60
check = [[0]*W for _ in range(H)]

ans = [S[0][0]]
que = [(0,0)]
check[0][0]=1
while que:
    que2 = []
    smin = INF
    for h,w in que:
        if h+1<H and check[h+1][w]==0:
            check[h+1][w]=1
            s = S[h+1][w]
            if s < smin:
                smin = s
            que2.append((s,h+1,w))
        if w+1<W and check[h][w+1]==0:
            check[h][w+1]=1
            s = S[h][w+1]
            if s < smin:
                smin = s
            que2.append((s,h,w+1))
    ans.append(smin)
    que = []
    for s,h,w in que2:
        if s==smin:
            que.append((h,w))

print("".join(map(chr, ans[:-1])))