Frequent error with shadertoys conversion

I often get an error with shadertoys conversion this is an example of the code :

void main() {

    vec2   c  = RENDERSIZE.xy; 
           gl_FragCoord.xy  += gl_FragCoord.xy - c;
    vec3   r  ;    
    float  e  = 0., t, d, m = TIME * .5;

the error in ISF editor is : Left-hand-side of assignment must not be read-only.
I tried to find any help on the internet but didn’t find a solution for this one.

this is the original shadertoys : Shader - Shadertoy BETA

any help ?