Cel/Toon shading limit colors after all light

Hello! I am new to writing shader so wrote this simple thing to cut light into 2 tones (cel/toon shading) but i wonder if it is possible to make it work better with multiple light sources. Here what i have now: http://i.piccy.info/i9/21ac64c2cfb212377de9e8a6d74cf8c2/1413491875/9992/721611/pic1.png I wonder if it possible to get result of all light sources together and then modify it so it it always stay in 1 of 2 values - light gray or dark gray. Something like this: http://i.piccy.info/i9/871e31f9ebd833b3558021a70ce68b14/1413492238/28967/721611/pic2.png So here is the code:

 Shader "Custom/Cel" {
     Properties {
         _MainTex ("Base (RGB)", 2D) = "white" {}
     }
     SubShader {
         Tags { "RenderType"="Opaque" }
         
         CGPROGRAM
         #pragma surface surf Toon
 
         sampler2D _MainTex;
 
         struct Input {
             float2 uv_MainTex;
         };
         
         void surf (Input IN, inout SurfaceOutput o) {
             half4 c = tex2D (_MainTex, IN.uv_MainTex);
             o.Albedo = c.rgb;
             o.Alpha = c.a;
         }
         
         half4 LightingToon (SurfaceOutput s, half3 lightDir, half atten) {
         half4 c;
         half dotProduct = dot(s.Normal, lightDir);
         dotProduct = floor(dotProduct * 0.5) * 0.5 + 0.5;
         c.rgb = s.Albedo * dotProduct;
         c.a = s.Alpha;
         return c;
         }
         ENDCG
     } 
     FallBack "Diffuse"

if anyone can give a solution, explanation how it works (even brief) will be greatly appreciated as i want to understand it.