import sys
input = sys.stdin.readline

H, W = map(int, input().split())
S = [input()[:-1] for _ in range(H)]
now = {(0, 0)}
ans = [S[0][0]]

for _ in range(H+W-2):
    m = 'z'
    nex = set()
    
    for x, y in now:
        if x+1<H:
            if m==S[x+1][y]:
                nex.add((x+1, y))
            elif m>S[x+1][y]:
                m = S[x+1][y]
                nex = {(x+1, y)}
        
        if y+1<W:
            if m==S[x][y+1]:
                nex.add((x, y+1))
            elif m>S[x][y+1]:
                m = S[x][y+1]
                nex = {(x, y+1)}
    
    ans.append(m)
    now = nex

print(''.join(ans))