The issue with
LL m = pow(2, n + 1) - 2;
is that pow(2, n + 1) is not a long long. It has the type double (refer to cppreference) and because the value is so large, subtracting 2 from it will not change its value. That means that m will not have the correct value. As you have already found, you need to assign the result first and then do the subtraction. Another alternative is to write your own pow that will return a integer type when given an integer type so you can do the raising to the power and subtraction at the same time.