from collections import defaultdict
h,w=map(int,input().split())
s=[]
for i in range(h):
  s.append(input())
a=[[0]*w for i in range(h)]
a[0][0]=1
dxdy=[(0,1),(1,0)]
ans=[]
for i in range(h+w-1):
  d=defaultdict(list)
  t=set()
  for x in range(max(0,i-h+1),min(i+1,w)):
    y=i-x
    if a[x][y]==1:
      d[s[x][y]].append((x,y))
      t.add(s[x][y])
  t=sorted(list(t))
  u=t[0]
  ans.append(u)
  for x,y in d[u]:
    for dx,dy in dxdy:
      if 0<=x+dx<h and 0<=y+dy<w:
        a[x+dx][y+dy]=1
print(*ans,sep='')