private static String getFixed(String expr) throws Exception {
        String result = expr.replaceAll("\\b" + "\\(" + "\\b", "*(");
        result = result.replaceAll("\\b" + "\\)" + "\\b", ")*");

        while(true){
            boolean isChanged = false;
            for(int i = 0 ; i < result.length() && !isChanged;i++){
                char symbol = result.charAt(i);
                if(symbol == '-' || symbol == '+' || symbol == '*' || symbol == '/'){
                    boolean isFound = true;
                    for(int j = i-1 ; j>= 0; j--){
                        if(result.charAt(j) == '('){
                            break;
                        }
                        if(result.charAt(j) >= '0' && result.charAt(j) <= '9' || result.charAt(j) == ')' ){
                            isFound = false;
                        }
                    }
                    if(isFound){
                        if(symbol  == '-' || symbol  == '+'){
                            result = result.substring(0,i) + "0" + result.substring(i);
                        } else {
                            throw new Exception("Operator on " + i + " position does not have any variable on left side");
                        }
                        isChanged = true;
                    }
                }
            }
            if (!isChanged) {
                break;
            }
        }
        return result;
    }