Shader error in 'Unlit/TestShader': out of memory while parsing

Unity Engine
1 
            fixed4 frag (v2f i) : SV_Target
            {
                half4 byte1 = half4(0,0,0,1);
                return half4(byte1.x,byte1.y,byte1.x,1);
            }

In HLSL, when using variable names such as byte1-4, the following error occurs:
‘Shader error in ‘Unlit/TestShader’: out of memory while parsing’