# 辞書順最小だから前から決めていく
# 斜め線上にあるアルファベットで最小のものを全部残しその次のものからも最小を選ぶ

H, W = map(int, input().split())
S = []
for i in range(H):
    temp = input()
    S.append(temp)
    
#print(S)

if H == 1:
    ans = S[0]
    print(ans)
    exit()
elif W == 1:
    ans = ''
    for i in range(H):
        ans += S[i]
    print(ans)
    exit()

usable = [[0]*W for h in range(H)]
usable[0][0] = 1

ans = S[0][0]
for d in range(1, H+W-1):
    if d < W:
        start = (0, d)
        length = min(H, d+1)
    else: 
        start = (d - (W-1), W-1)
        length = H - (d - (W-1))
    #print('d', d, 'start', start, 'length', length)
    smallest = 'zz' #辞書順最悪ダミー
    smallest_index = []
    for i in range(length):
        if (0 <= start[0]+i-1 < H and 0 <= start[1]-i < W and usable[start[0]+i-1][start[1]-i] == 1) or (0 <= start[0]+i < H and 0 <= start[1]-i-1 < W and usable[start[0]+i][start[1]-i-1] == 1):
            letter = S[start[0]+i][start[1]-i]
            if letter == smallest:
                smallest_index.append(start[0]+i)
            elif letter < smallest:
                smallest = letter
                smallest_index = []
                smallest_index.append(start[0]+i)
    #print(smallest, smallest_index)
    ans += smallest
    
    for i in range(length):
        if start[0]+i in smallest_index:
            usable[start[0]+i][start[1]-i] = 1
    #print(usable)
    #print()
    
print(ans)