h,w=map(int,input().split())
s=[]
for i in range(h):
  s.append(input())
dxdy=[(0,1),(1,0)]
ans=[]
todo=[(0,0)]
for i in range(h+w-1):
  d=[]
  mn='z'
  for x,y in todo:
    if mn==s[x][y]:
      d.append((x,y))
    elif mn>s[x][y]:
      d=[(x,y)]
      mn=s[x][y]
  ans.append(mn)
  todo=[]
  ok=[0]*h
  for x,y in d:
    for dx,dy in dxdy:
      if 0<=x+dx<h and 0<=y+dy<w:
        ok[x+dx]=1
  for x in range(h):
    if ok[x]:
      todo.append((x,i+1-x))
print(*ans,sep='')