Integral approximation with midpoint method

I want to write a code for the Integral approximation with the midpoint method.

Mathematically, I was thinking like this: y'(t)=f(t,y(t))=-y(t)

The rectangle rule: y(t + h)=y(t) + h · f(t+h/2,y(t)+h/2*f(t,y(t)))

for:

h=0.5 and y(0)=1 (t0=0, y(t0)=1)

I would like to calculate the next step: t1=t0+h=0.5, y(t1)=?

           y(t1)=y(t0+h)=y(t0)+h*f(t0+h/2,y(t0)+h/2*f(t0,y(t0)))=

                        =1+0.5*f(0.25,1+0.25*f(0,1))=

                        =1+0.5*f(0.25,1+0.25*(-1))=

                        =1+0.5*f(0.25,0.75) = 1+0.5*(-0.75)=0.625

I don't know how to represent the function f in Matlab (syms ?) so that it would know to calculate f(0,1) for example.

Can someone help me, please?