Substitute not-number value with Panda not working for all

Question

I want to read values form csv with Panda. I have float columns like this in test.csv

a;b;c
1.4;2.4;-
2.3;4.9;3.8
3.1;5.3;2.7
6.6;-;-
9.6;n/a;3.8
2.3;3.4;6.3