I made a simple Feynman diagram of the channel S=(k1 + k2)^2 in the Positron - Photon scattering as follows 

```
\feynmandiagram[horizontal=a to b]{
    i1 [particle=\(\gamma\)] 
        -- [photon, momentum=\(\vec{k}_1\)] a -- [fermion, momentum'=\(\vec{k}_2\)] i2 [particle=\(e^{+}\)],
    a -- [anti fermion, edge label'=\((\vec{k}_1+\vec{k}_2)\)] b,
    f1 [particle=\(\gamma\)] 
        -- [photon, momentum'=\(\vec{k}_3\)] b -- [anti fermion, momentum=\(\vec{k}_4\)] f2 [particle=\(e^{+}\)],
};

```

The problem is that the k2 and k3 arrows are not in the correct direction. How may I do it to get the correct one? I do not get how to build this S channel properly.