As Ralph writes sum(TOTAL c2) gives the total for the dimension. But you have these options:
// Get the total for ALL values, disregarding any selections:
sum(ALL c2)
// Get the total for all values in the Dimension using selections:
sum(TOTAL c2)
//Get the sum for each value of c1, even when a value is selected (this is similar to sum(c2) when nothing is selected:
sum({<c1= >} c2)
You can then combine the above to get various results.