How to calculate the intensity of the interference of two waves in a given point?

There are two different point sources which produce spherical waves with the same power, amplitude, ω, wavenumber and phase.

I can calculate the intensity of each wave in a point:

$$I_1 = P / (4 \pi r_1^2)$$ $$I_2 = P / (4 \pi r_2^2)$$ But how can I calculate the intensity of the resulting wave in that point?