How to make a number higher than Quintillion? (1000000000000000000)

Hey guys i really need your help! I got a converting script which converts Numbers like 1.000.000 to 1 Million. But when i try to add a new number higher than Quintillion it says "Integral constant is too large". (trying to make a clicking game) how can i make my number higher than Quintillion? :( Thanks for your Help, here is the script:

public class CurrencyManager : MonoBehaviour {

 private static CurrencyManager instance;
 public static CurrencyManager Instance {
     get {
         return instance;
     }
 }

 void Awake()
 {
     CreateInstance ();
 }


     void CreateInstance(){
     if (instance == null) {
         instance = this;
     }
 }
     
 public string GetCurrencyIntoString(float valueToConvert, bool currencyPerSec, bool currencyPerClick ){
     string converted;

     if (valueToConvert >=   1000000000000000000000)
     {
         converted = (valueToConvert / 1000000000000000000000f).ToString("f3") + " SEX";
     }else if (valueToConvert >= 1000000000000000000) {
         converted = (valueToConvert / 1000000000000000000f).ToString ("f3") + " QUI";
     } else if (valueToConvert >= 1000000000000000) {
         converted = (valueToConvert / 1000000000000000f).ToString ("f3") + " QUA";
     } 
     else if (valueToConvert >= 1000000000000) {
         converted = (valueToConvert / 1000000000000f).ToString ("f3") + "T";
     }
     else if (valueToConvert >= 1000000000) {
         converted = (valueToConvert / 1000000000f).ToString ("f3") + "B";
     }
     else if (valueToConvert >= 1000000) {
         converted = (valueToConvert / 1000000f).ToString ("f3") + "M";
     }
     else if (valueToConvert >= 999) {
         converted = (valueToConvert / 999f).ToString ("f3") + "K";
     }
     else if (valueToConvert <= 1000)
     {
         converted = Mathf.Round(valueToConvert).ToString();
     }

}