How to do simple shader

Suppose I have a NxN texture.

For each pixel (i,j), if i+j is even, I want that pixel to be black, otherwise it must be white.

How can I do such shader?

 

Code (CSharp):

1. Shader "Debug/UV 1" {
2. SubShader {
3.     Pass {
4.         CGPROGRAM
5.         #pragma vertex vert
6.         #pragma fragment frag
7.         #include "UnityCG.cginc"
8.  
9.         // vertex input: position, UV
10.         struct appdata {
11.             float4 vertex : POSITION;
12.             float4 texcoord : TEXCOORD0;
13.         };
14.  
15.         struct v2f {
16.             float4 pos : SV_POSITION;
17.             float4 uv : TEXCOORD0;
18.         };
19.        
20.         v2f vert (appdata v) {
21.             v2f o;
22.             o.pos = UnityObjectToClipPos(v.vertex );
23.             o.uv = float4( v.texcoord.xy, 0, 0 );
24.             return o;
25.         }
26.        
27.         half4 frag( v2f i ) : SV_Target {
28.             if ((i.x == i.y))
29.                 return 0;
30.             else return 1;
31.  
32.         }
33.         ENDCG
34.     }
35. }
36. }

Like this?

Haven't tested yet.

 

who is i.x and i.y? there isn't definition for them in struct v2f

anyways I think this is wrong, you are using (x == y) but it should be something like (x + y == 2n)

 

Code (CSharp):

1.  
2. v2f vert (appdata v) {
3.             v2f o;
4.             o.pos = UnityObjectToClipPos(v.vertex );
5.             o.uv = float4( v.texcoord.xy, 0, 0 );
6.             return o;
7.         }
8.      
9.         half4 frag( v2f i ) : SV_Target {
10.             float current = tex2D(_Texture, i.uv).r; // red channel holds relevant data
11.            float next = tex2D(_Texture, i.uv + float2(1/_TextureSize, 0)).r;
12.           float remainder = fmod(current + next, 2);
13.  
14.          return remainder == 0 ? 1 : 0;
15.         }
16.  

The above is code pointing in the general direction.
You need to define a sampler for your texture and probably a property, too (named _Texture in the above code).
1/_TextureSize for example is something that you can get from internal Unity variables which I do not know the name of at the moment - I made it up. Also: 1/_TextureSize might not end up in the middle of the texel, so you may need to adjust it with a halftexel offset.
The returned term at the end of the fragment shader needs to be of float4 - in my case it is just a scalar value.
Hope this helps to get you started.