How to increase value of map in tact (TON blockchain)?

I am trying to find a way to increase value of map, here is my code

import "@stdlib/deploy";


contract Holders with Deployable {
    holders: map<Address, Int>;
    owner: Address;

    init() {
        self.owner = sender();
    }

    receive("start") {
       let amount: Int? = self.holders.get(sender());
       self.holders.set(sender(), amount + 100);
    }  
     
    get fun balance(key: Address): Int? {
        return self.holders.get(key);
    }
}

I must type as 'Int?' of amount variable's state, if I not type like it gives error, but if I type as 'Int?' it gives error for addition at 'amount + 100'.

Is there a way to increase value of map?

If you sure that key is exists in map you can use !! operator to cast Int? to Int

let amount: Int? = self.holders.get(sender());
if (amount != null) {
  self.holders.set(sender(), amount!! + 100);
}

It should works fine