First compile like this :

javac Main.java

Then run the program like this:

java Main mul 10 20 (does a multiplication, the russian way)

java Main eu 146 46 (finds the GCD with the euclides algorithm)

java Main tri 5 2 7 (tells you if 3 sides (a, b and c) can form a triangle, and if yes, it gives you its type)