Smallest value for n in Leibniz Formula for Pi

Hello, I am new to MATLAB and am having some trouble to solve this problem. Given the Leibniz Formula for Pi:

In which A is the result for a given N, and R becomes Pi as N increases:

Here it is defined the relative error E=|(A-R)/R)|. What I need to look for is the smallest value for N in the case of E<=10＾-6. This is what I`ve been trying in matlab so far:

What I get is a list for all the values for A and R, as well as ans=[]. but I don`t know how to ask exactly for the value of N for my desired E. I`m sorry if this is a stupid question, like I said I just started using matlab. Any help would be appreciated.

Answer 1

[Torsten]

a = 0;
rel_error = 1.0;
n = 0;
while (rel_error > 1.0e-6)
  a = a + 4*(1-2*mod(n,2))/(2*n+1);
  rel_error = abs(a-pi)/pi;
  n = n+1;
end
n