h, w = map(int, input().split())
s = [input() for _ in range(h)]
ans = [s[0][0]]
tij = {(0, 0, s[0][0])}
m = s[0][0]
for _ in range(h+w-2):
    tij_new = set()
    m_new = "z"
    for i, j, t in tij:
        if t == m and i < h-1:
            tij_new.add((i+1, j, s[i+1][j]))
            m_new = min(m_new, s[i+1][j])
        if t == m and j < w-1:
            tij_new.add((i, j+1, s[i][j+1]))
            m_new = min(m_new, s[i][j+1])
    tij = set(tij_new)
    m = m_new
    ans.append(m)
print("".join(ans))