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=[[] for i in range(26)]
    
    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]
                D[ord(k)-97].append((z,w))

    for i in range(26):
        if D[i]!=[]:
            ANS.append(chr(i+97))
            break
            
    Q=[]
    for to in D[i]:
        Q.append(to)

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