import sys
input = sys.stdin.readline

H,W=map(int,input().split())
S=[input().strip() for i in range(H)]

Q=[(0,0)]

ANS=[S[0][0]]

while Q:
    D=dict()
    
    while Q:
        x,y=Q.pop()

        for z,w in [(x+1,y),(x,y+1)]:
            if 0<=z<H and 0<=w<W:
                k=S[z][w]
                if not(k in D):
                    D[k]=([(z,w)])
                else:
                    D[k].append((z,w))

    if list(D)==[]:
        break

    MIN=min(D)
    ANS.append(MIN)
    Q=[]

    for to in D[MIN]:
        Q.append(to)

    Q=list(set(Q))
        
print("".join(ANS))