Steps:
1. Cut the rope into two parts, 50 mts and 25mts
2. Tie the 25 mts at the top of the building and hold the
other part with you.
3. Using the tied 25mts rope descend.
4. After reaching the other end, tie that end.
5. Now ascend to the top of the buiding using the same 25mts
rope and untie it. you will  now fall from there to 50 mts
(remember the thread is tied at 25 mts)
6. Use the remaining 50 mts rope left with you to descend
completely
DONE