Summing together same-key items from a multimap?

I'm aware this has been asked on here before:

stl::multimap - how do i get groups of data?

but the answer:

pair<Iter, Iter> range = my_multimap.equal_range("Group1");
int total = accumulate(range.first, range.second, 0);

does not compile for me, when I do:

pair<multimap<int32_t, float>::iterator, multimap<int32_t, float>::iterator>> range = multimap.equal_range(an_int);
float total = accumulate(range.first, range.second, 0);

This was on GCC 4.8

Could anyone confirm whether they can compile this/offer the correct answer?