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