import random
h,w = map(int,input().split())
S = [list(input()) for _ in range(h)]

Ans = 'z'*10000
for _ in range(10**4):
    ans = S[0][0]
    a,b = 0,0
    for _ in range(h+w):
        if a == h-1 and b == w-1:
            if ans < Ans:
                Ans = ans
            else:
                break
        else:
            if a+1 == h:
                ans += S[a][b+1]
                b += 1
            elif b+1 == w:
                ans += S[a+1][b]
                a += 1
            else:
                if S[a+1][b] > S[a][b+1]:
                    ans += S[a][b+1]
                    b += 1
                elif S[a+1][b] < S[a][b+1]:
                    ans += S[a+1][b]
                    a += 1
                else:
                    R = random.randint(0,1)
                    if R == 1:
                        ans += S[a][b+1]
                        b += 1
                    else:
                        ans += S[a+1][b]
                        a += 1
print(Ans)