H,W = map(int,input().split())
S = [input().strip() for _ in range(H)]
ans = {(0,0)}
T = S[0][0]
while True:
    i,j = ans.pop()
    if i+j==H+W-2:break
    ans.add((i,j))
    ans1 = set()
    t = "z"
    for i,j in ans:
        if i==H-1:
            if S[i][j+1]<t:
                t = S[i][j+1]
                ans1 = {(i,j+1)}
            elif t==S[i][j+1]:
                ans1.add((i,j+1))
        elif j==W-1:
            if S[i+1][j]<t:
                t = S[i+1][j]
                ans1 = {(i+1,j)}
            elif t==S[i+1][j]:
                ans1.add((i+1,j))
        else:
            if S[i][j+1]<t:
                t = S[i][j+1]
                ans1 = {(i,j+1)}
            elif t==S[i][j+1]:
                ans1.add((i,j+1))
            if S[i+1][j]<t:
                t = S[i+1][j]
                ans1 = {(i+1,j)}
            elif t==S[i+1][j]:
                ans1.add((i+1,j))
    ans = ans1
    i,j = ans.pop()
    T = T+S[i][j]
    ans.add((i,j))
print(T)