Using Java 8 streams how do I convert Map<String, List<Something>> to Map<String, Integer>?

So, I want to make a new map from the old one. In the new Map<String, Integer> the String key denotes the same key as in the first map and the Integer value denotes the size of the corresponding List<Something>. I want to make the transformation using Java 8 streams. I tried something like, but it gives me errors.

Map<String, List<Something>> map1=.....
Map<String, Integer> map2;
map2=map1.entrySet().stream().collect(Collectors.toMap(p-> p.getKey(), p->getValue().size()));