Procedure:
Consider the power place values ofthe number from right to left is 0,1,2,3,4,5........ so on
Calculate the place value of first 1 of the number from left where contineous 1s are there to the left of it till end(Let called the place i).
Then do this formula: -(2^i)+(sum of all 2^j) Here j is from i-1 to 0
Here Number is:1111111111110101
-(2^4) +2^2+2^0=-16+4+1=-11