Lambdas do not work in Unity

Why expressions like this will not work in Unity?

Func<int, int> hi = (int a) => a * 2;

I’ve been able to achieve something similar using Actions

Action <int, int> hi = (a,b) => someFunc(a,b);

I tried this, but it didn’t work, maybe there’s a better way to write it:

Action <int, int> hi = (a,b) => a*b;