Calculating the total from identical tags

Please advise on how I achieve the following transformation.

The input xml I have is of the format.

 <Employees>
     <Employee>
            <Name>Don</Name>
            <Salary>5000</Salary>
            <Expense>1000</Expense>
            <Expense>500</Expense>
            <Expense>300</Expense>
    </Employee>
     <Employee>
            <Name>John</Name>
            <Salary>5000</Salary>
            <Expense>100</Expense>
            <Expense>400</Expense>
            <Tax>500</Tax>
            <Tax>200</Tax>
    </Employee>

And the output should be

<Employees>
     <Employee>
            <Name>Don</Name>
            <Salary>5000</Salary>
            <Expense>1800</Expense>
    </Employee>
     <Employee>
            <Name>John</Name>
            <Salary>6000</Salary>
            <Expense>500</Expense>
            <Tax>700</Tax>
    </Employee>

Thanks