h,w = map(int, input().split())
Alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
alpha = "abcdefghijklmnopqrstuvwxyz"
Dict_alpha = {}
dict_alpha = {}
for i in range(26):
    Dict_alpha[Alpha[i]] = i
    dict_alpha[alpha[i]] = i
Dec = lambda x:Dict_alpha[x]
dec = lambda x:dict_alpha[x]
s = [list(map(dec, list(input()))) + [26] for i in range(h)] + [[26] * w]
#print(s)
st = [(0,0)]
ans = [alpha[s[0][0]]]

for i in range(h+w-2):
    mn = 100
    nst = []
    for jx, jy in st:
        t = s[jx+1][jy]
        if t == mn:
            nst.append((jx+1, jy))
        elif t < mn:
            mn = t
            nst = [(jx+1, jy)]
            
        t = s[jx][jy+1]
        if t == mn:
            nst.append((jx, jy+1))
        elif t < mn:
            mn = t
            nst = [(jx, jy+1)]
    st = nst
    #print(mn)
    ans.append(alpha[mn])

print("".join(ans))