A source and a detector move away from each other in absence of wind with a speed of $20 \mathrm{~m} / \mathrm{s}$ with respect to the ground. If the detector detects a frequency of 1800 Hz of the sound coming from the source, then the original frequency of source considering speed of sound in air $340 \mathrm{~m} / \mathrm{s}$ will be $\_\_\_\_$ Hz.