Minimum Deletions to make string balanced

'''You are given a string s consisting only of characters 'a' and 'b'.

You can delete any number of characters in s to make s balanced. s is balanced if there is no pair of indices (i,j) such that i < j and s[i] = 'b' and s[j]= 'a'.

Return the minimum number of deletions needed to make s balanced.'''

class Solution(object):
    def minimumDeletions(self, s):
        """
        :type s: str
        :rtype: int
        """
        min_del = 0
        bcount = 0
        i = 0
        for i in range(len(s)):
            if s[i]=='a':
                min_del = min(min_del+1, bcount)
            else:
                bcount += 1
        return(min_del)