runge kutta method wont plot graph

hi all i am having some issues plotting a runge kutta method ode. the general ode is

da/dz=0.5beta2*x1*x4 where the initial conditions of x1 and x4 are 0.23 and 0.0079 respectivily and beta2 has a value of 17 this is the code i have done so far

close all 
%define constants
%equation to plot is dx1/dz =0.5*beta2*x1*x4
%initial conditions are x1=0.23 and x4=-0.0079
%time=1.422*10^-15;
%w=1/time;
%n2=2.7*10^-20;
%Aeff=80*10^-6;
%c=340;
beta2=17*10^-6;

%a=x1,b=x4
%defines function
f=@(a,b)0.5*beta2*a*b;

%initial conditions taken from paper

a(1)=0.23;
b(1)=-0.0079;

%stepsize
h=0.5;

z=linspace(0,1000,1000);
N=ceil(z/h);
for i=1:N
     k1=f(a(i), b(i))
     k2=f(0.5*h,a(i)+0.5*k1,b(i)+0.5*k1)
     k3=f(0.5*h,a(i)+0.5*k2), b(i)+0.5*k2
     k4=f(0.5*h,a(i)+0.5*k3, b(i)+0.5*k3)
     y(i+1)=(1/6)*k1+(1/3)*k2+(1/3)*k3+(1/6)*k4
end
plot(z,a)

Answer 1

[Stalin Samuel]

here you can find the example code