h,w = map(int,input().split())
s = [list(input()) for _ in range(h)]
vis = [[-1] * w for _ in range(h)]
vis[0][0] = 0
vecs = [(0,1),(1,0)]
mark = s[0][0]
ans = ["" for _ in range(h+w)]
for n in range(h+w):
    n_mark = '{'
    for i in range(n+1):
        j = n-i
        if not (0 <= i <= h-1 and 0 <= j <= w-1):continue
        if s[i][j] == mark and vis[i][j] == 0:
            vis[i][j] = 1
            ans[n] = mark
            for vi,vj in vecs:
                ni,nj = i + vi,j + vj
                if not (0 <= ni <= h-1 and 0 <= nj <= w-1):continue
                vis[ni][nj] = 0
                if n_mark > s[ni][nj]:
                    n_mark = s[ni][nj]
                    
    mark = n_mark

print("".join(ans))