How to calculate maximum unambiguous range for MFCW radar given F1, F2 frequencies?

From the book i read now, range is estimated by R=(C*phi)/(4*Pi*modulationFreq), where phi - is phase difference. I need to calculate maximum distance using this formula. It can be putting maximum (phi) value to the formula. But I can't understand is it Pi or 2*Pi?

Rmax=(C*Pi)/(4*Pi*modulationFreq) or
Rmax=(C*2*Pi)/(4*Pi*modulationFreq)

C - speed of light
modulationFreq - difference between two frequencies
R - range in meters

For single-frequency CW distance is unambiguous up to Pi. So i guess here is the same?

Thanks for reading!