I want to fade out this Texture Masking Shader (using Rendertextures) and I need your help

Thanks for your reply. Unfortunately, when I put the "fixed4 _Alpha" underneath CGPROGRAM like this:

 Shader "TextureMasking" {
     Properties {
         _MainTex ("Main", 2D) = "white" {}
         _MaskTex ("Mask", 2D) = "white" {}
         _Alpha ("Alpha", Float) = 0.5
     }
 
     SubShader {
         Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent"}
         ZWrite Off
         ZTest Off
         Blend SrcAlpha OneMinusSrcAlpha
         Pass {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #pragma fragmentoption ARB_precision_hint_fastest
             #include "UnityCG.cginc"
 
             uniform sampler2D _MainTex;
             uniform sampler2D _MaskTex;
             uniform float4 _MainTex_ST;
             uniform float4 _MaskTex_ST;
             uniform fixed4 _Alpha;
 
             struct app2vert
             {
                 float4 position: POSITION;
                 float2 texcoord: TEXCOORD0;
             };
 
             struct vert2frag
             {
                 float4 position: POSITION;
                 float2 texcoord: TEXCOORD0;
             };
 
             vert2frag vert(app2vert input)
             {
                 vert2frag output;
                 output.position = mul(UNITY_MATRIX_MVP, input.position);
                 output.texcoord = TRANSFORM_TEX(input.texcoord, _MainTex);
                 return output;
             }
             
             fixed4 frag(vert2frag input) : COLOR
             {
                 fixed4 main_color = tex2D(_MainTex, input.texcoord);
                 fixed4 mask_color = tex2D(_MaskTex, input.texcoord);
                 
                 return fixed4(main_color.r, main_color.g, main_color.b, main_color.a * (_Alpha - mask_color.a));
             }
             
             ENDCG
         }
     }
 }

It gives me an error:

Shader error in 'TextureMasking': incorrect number of arguments to numeric-type constructor at line 52 (on d3d11)

It seems to only accept the _Alpha variable when it's inside the frag, but then it's bugging out..