import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################
h, w = na()

dist = ["{"] * (h + w - 1)
s = [input() for i in range(h)]
from collections import deque

d = deque([(0, 0, s[0][0])])
dist[0] = s[0][0]
f = [[0] * w for i in range(h)]
while d:
    x, y, z = d.popleft()
    if dist[x+y] != z or f[x][y]:
        continue
    f[x][y] = 1
    #print(x,y,z)
    nx = x + 1
    ny = y
    if nx < h and ny < w and s[nx][ny] <= dist[nx+ny]:
        dist[nx+ny] = s[nx][ny]
        d.append((nx,ny,dist[nx+ny]))
    nx = x
    ny = y + 1
    if nx < h and ny < w and s[nx][ny] <= dist[nx+ny]:
        dist[nx+ny] = s[nx][ny]
        d.append((nx,ny,dist[nx+ny]))

print("".join(dist))