How do you add up variables?

I was doing a code when I faced this doubt: How do I change initial a value from 0 to 1?

void setup (){

pinMode(4, INPUT_PULLUP);

void loop(){

int a=0;

if (digitalRead(3) == LOW){
int a +1;

if (a==1 && digitalRead(3) == LOW){
tone (0,3200); // is the buzzer making any sound

When I press the button 3 twice, no sound sounds. Whats wrong with my code?? (srry if its something very obvious, im a noob)

use a=a+1;

Please use code tags (</> button on the toolbar) when you post code or warning/error messages. The reason is that the forum software can interpret parts of your code as markup, leading to confusion, wasted time, and a reduced chance for you to get help with your problem. This will also make it easier to read your code and to copy it to the IDE or editor. If your browser doesn’t show the posting toolbar then you can just manually add the code tags:
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Please always do a Tools > Auto Format on your code before posting it. This will make it easier for you to spot bugs and make it easier for us to read. If you’re using the Arduino Web Editor you will not have access to this useful tool but it’s still unacceptable to post poorly formatted code. I recommend you to use the standard IDE instead.

As posted your code does not even compile. Perhaps you could post the version that you successfully uploaded and tested.