Frog in the Well

First day it jumps 5 meters and falls 4 meters. Now it will be at a height of 1 meter.
Second day it jumps to 6 meters and falls back to 2 meter.
Third day it jumps to 7 meters and falls back to 3 meter.
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Fifteenth day it jumps to 19 meters and falls back to 15 meters.
Finally, sixteenth day it jumps to 20 meters, but now the frog have reached the top of the well.
So the Frog reaches the top of the well at the sixteenth day.