Get Refresh Token by giving Consumer Secret and Consumer Key

RefreshTokenVFSLDS.html

<apex:page controller="RefreshTokenClass" >
    <apex:slds />
    <apex:form >
        <div class="slds-form slds-form_stacked">
            <div class="slds-form-element slds-size_2-of-4">
                <label class="slds-form-element__label" for="input-id-01"><b>Client Id</b></label>
                <div class="slds-form-element__control">
                   <apex:inputText id="clienti" styleClass="slds-input" value="{!clientid}"/>
                </div>
            </div>
            <div class="slds-form-element slds-size_2-of-4">
                <label class="slds-form-element__label" for="input-id-01"><b>Client Secret</b></label>
                <div class="slds-form-element__control">
                   <apex:inputText id="clientsecr" styleclass="slds-input" value="{!clientsecret}" />
                </div>
            </div>
        </div>
        <br/>
        <apex:commandButton action="{!auth_step_1}" value="Step1:Get Authorization Code"/>&nbsp;
        <apex:commandButton action="{!auth_step_2}" value="Step2:Get RefreshToken"/>
        <br/>
        <div style="display:{!IF($CurrentPage.parameters.code != Null && str = Null ,'block','none')}">
            <label><b>Authorization Code:</b></label>
            <div class="slds-page-header slds-p-top_medium">
                <b>{!$CurrentPage.parameters.code}</b> 
            </div> 
        </div>
        <div style="display:{!IF(str = Null, 'none','block')}">
            <label><b>Refresh Token:</b></label>
            <div class="slds-page-header slds-p-top_medium" style="display:{!IF(str = Null,'none','block')}">
                <b>{!str}</b> 
            </div>
        </div>
    </apex:form>
</apex:page>