PROBLEM: https://code.google.com/codejam/contest/11274486/dashboard#s=p2 (JANE FLOWERSHOP)

i am getting wrong answer on large subtask. i used similar concept as binary search and went on digging near to the root of the equation using property that f(x) has a root between a and b if f(a)*f(b)<0 . it works for sample cases also. code: https://gist.github.com/ankesh007/f9e44d3cdf8ed5d8e3e218167c6272a1

please help/share approach.