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

    T=[chr(S[0][0])]
    Y={(0,0)}

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

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

print(solve())