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

dp = {(0, 0)}
ans = S[0][0]
for i in range(H+W-2):
    ndp = set()
    mins = 'zz'
    for h, w in dp:
        if h < H-1:
            nt = S[h+1][w]
            if mins == nt:
                ndp.add((h+1, w))
            elif mins > nt:
                ndp = {(h+1, w)}
                mins = nt
        if w < W-1:
            nt = S[h][w+1]
            if mins == nt:
                ndp.add((h, w+1))
            elif mins > nt:
                ndp = {(h, w+1)}
                mins = nt
    dp = ndp
    ans += mins

print(ans)