Picking a random element from STL multimap

[Steve]

Hello,

when a multimap has multiple elements for a certain key, I want to
pick any random one of them.
I use equal_range to get the bounds iterators, and then try to add a
random number to the lower iterator, but the compiler complains that
there is no operator+.

map<char, int> myMap;

(... fill map with elements)

pair<multimap<char,int>::iterator,multimap<char,int>::iterator> ret =
myMap.equal_range('a');
multimap<char,int>::iterator firstIter = ret.first;
long numMatches = myMap.count('a');
int r = (random number between 0 and numMatches);
multimap<char,int>::iterator randomIter = firstIter +r;


I'm sure I've used +/- on iterators before, but maybe they only work
on other containers?

Is there a way to achieve this? I'm happy to go a completely different
route if there's a more efficient way.

Thanks

Steve