private int checkPriorityOperations(char aStr){
        switch (aStr){
            case 'm':
                return 3 ;
            case '^':
                return 3 ;
            case 's':
                return 3 ;
            case '+':
                return 0 ;
            case '-':
                return 0 ;
            case '/':
                return 1 ;
            case '*':
                return 1 ;
        }
        return -1;
    }