Longest K unique characters substring

#User function Template for python3

class Solution:

    def longestKSubstr(self, s, k):
        d={}
        n=len(s)
        l=0
        ans=0
        for r in range(n):
            while len(d)>k:
                if d[s[l]]==1:
                    d.pop(s[l])
                else:
                    d[s[l]]-=1
                l+=1
            d[s[r]]=1+d.get(s[r],0)
            if len(d)==k:
                ans=max(ans,r-l+1)
        return ans if ans else -1