Billboard shader lock roll(z) rotation

Hello! I need lock Z-follow rotation (roll) for this billboard shader. How to do this? Please help.

 Shader "Billb Alpha Blend" 
 {
    Properties {
      _Color ("Color", Color) = (1,1,1,1)
         _MainTex ("Texture Image", 2D) = "white" {}
    }
 
    SubShader {
         
       Tags {"Queue" = "Transparent"}
 
       Pass {
 
          ZWrite Off
 
          Blend SrcAlpha OneMinusSrcAlpha
 
          CGPROGRAM
  
          #pragma vertex vert  
          #pragma fragment frag 
             
          uniform sampler2D _MainTex;  
  
          struct vertexInput {
             float4 vertex : POSITION;
             float4 tex : TEXCOORD0;
          };
          struct vertexOutput {
             float4 pos : SV_POSITION;
             float4 tex : TEXCOORD0;
          };
  
          vertexOutput vert(vertexInput input) 
          {
             vertexOutput output;
  
             output.tex = input.tex;
             output.pos = mul(UNITY_MATRIX_P, mul(UNITY_MATRIX_MV, float4(0.0, 0.0, 0.0, 1.0)) - float4(input.vertex.x, input.vertex.z, 0.0, 0.0));
             return output;
          }
          
          fixed4 _Color;
          
          float4 frag(vertexOutput input) : COLOR
          {
             return tex2D(_MainTex, float2(input.tex.xy)) * _Color;
          }
  
          ENDCG
       }
    }
 }