h, w = map(int, input().split())
S = [input() for _ in range(h)]
def f(i, j):
    return i * w + j
ans = [S[0][0]]
se = {0}
for _ in range(h + w - 2):
    nex = set()
    mi = "z"
    for p in se:
        i = p // w
        j = p - i * w
        if i != h - 1:
            s = S[i + 1][j]
            if s < mi:
                mi = s
                nex = {f(i + 1, j)}
            elif s == mi:
                nex.add(f(i + 1, j))
        if j != w - 1:
            s = S[i][j + 1]
            if s < mi:
                mi = s
                nex = {f(i, j + 1)}
            elif s == mi:
                nex.add(f(i, j + 1))
        
    ans.append(mi)
    se = nex
print(*ans, sep="")