f(n) = [itex]\frac{z^n * s}{100^n}[/itex] + 2*[itex]\frac{z^(n-1) * s}{100^(n-1)}[/itex] + [itex]\frac{z^(n-2) * s}{100^(n-2)}[/itex] + [itex]\frac{z^(n-3) * s}{100^(n-3)}[/itex] + [itex]\frac{z^(n-4) * s}{100^(n-4)}[/itex]

I stopped at this amount of terms arbitrarily, but really I want the series to stop adding terms when n is equal to x in "z^(n-x)" and "100^(n-x)". How can I say that in a general form?