H,W = map(int, input().split())
S = []
for _ in range(H):
    S.append(list(input()+'~'))
S.append(list('~'*W))

pos = set()
pos.add(0)
ANS = [S[0][0]]

while len(ANS)<H+W-1:
    alphabet = 'z'
    nextPos = set()
    for xy in list(pos):
        x,y = xy%W,xy//W
        if S[x+1][y]==alphabet:
            nextPos.add(xy+1)
        elif S[x+1][y]<alphabet:
            alphabet = S[x+1][y]
            nextPos = set()
            nextPos.add(xy+1)
        if not xy+W in nextPos:
            if S[x][y+1]==alphabet:
                nextPos.add(xy+W)
            elif S[x][y+1]<alphabet:
                alphabet = S[x][y+1]
                nextPos = set()
                nextPos.add(xy+W)
    ANS.append(alphabet)
    pos = nextPos
print(''.join(map(str, ANS)))