h,w = map(int,input().split())
s = [list(input()) for _ in range(h)]
from collections import defaultdict
d = defaultdict(list)
for i in range(h):
    for j in range(w):
        d[i+j].append((i,j))
        
vis = [[0] * w for _ in range(h)]
vis[0][0] = 1
vecs = [(0,1),(1,0)]
for key in sorted(d.keys()):
    if key == h + w - 2 :break
    mark = chr(123)
    for i,j in d[key]:
        if vis[i][j] == 1:
            for vi,vj in vecs:
                ni,nj = i + vi,j + vj
                if not (0 <= ni <= h-1 and 0 <= nj <= w-1):continue
                if mark > s[ni][nj]:
                    mark = s[ni][nj]
    for i,j in d[key]:
        if vis[i][j] == 1:
            for vi,vj in vecs:
                ni,nj = i + vi,j + vj
                if not (0 <= ni <= h-1 and 0 <= nj <= w-1):continue
                if mark == s[ni][nj]:
                    vis[ni][nj] = 1
ans = [s[h-1][w-1]]
from collections import deque
vecs = [(0,-1),(-1,0)]
i,j = h-1,w-1
while (i+j):
    for vi,vj in vecs:
        ni = i+vi
        nj = j+vj
        if not (0 <= ni <= h-1 and 0 <= nj <= w-1):continue
        if not vis[ni][nj]:continue
        i ,j = ni,nj
        ans.append(s[ni][nj])
print("".join(ans[::-1]))