def solve():
    H,W=map(int,input().split())
    S=[]
    for _ in range(H):
        S.append(list(input()))

    T=[S[0][0]]
    Y={0}

    K=10000

    for _ in range(H+W-2):
        X=list(Y)
        Y=set()

        c=chr(ord("z")+1)
        for k in X:
            i,j=divmod(k,K)
            if i+1<H and S[i+1][j]<c:
                c=S[i+1][j]
            if j+1<W and S[i][j+1]<c:
                c=S[i][j+1]
        for k in X:
            i,j=divmod(k,K)
            if i+1<H and S[i+1][j]==c:
                Y.add((i+1)*K+j)
            if j+1<W and S[i][j+1]==c:
                Y.add(i*K+(j+1))
        T.append(c)
    return "".join(T)

print(solve())