Assume that the shark has gone till he has reached the square number n and there, his strength has became 0 .

We know that n have to be divisible by 2 . The number of squares that are divisible by 2 and <= n is floor ( n/2 ) .

The number of squares divisible by 42 is floor ( n / 42 ) .

Here we want the number of squares divisible by 2 and not divisible by 42. Though we have floor (n/2) - floor(n/42) = s .

To solve this equation, you can write it in the form without floor functions (( n/2 + n/42 = s )) .

Then solve this equation. Let the answer to be x. First take the integer part of x.

Then set x as x-10. At last, run a loop from 0 to 20 and in each iteration, increment x.

After incrementing, check if x%2 ==0 and floor(x/2)-floor(x/42) == s .

The first x that satisfies the constraints is the answer.

Sorry for my bad English.