Try this test:
10
e
e
f
f
unknown
g
unknown
g
d
d
Answer: Igor is wrong. But my programm gave me "5", because i forgot check v[j] when last="unknown".

if (last == "unknown" && v[j] != "unknown")
                    {
                        last = v[j];
                        if (m[last] == true) f = false;
                    }