from collections import Counter

def anagram(s):
    n = len(s)
    if n % 2 == 1:
        return -1  
    a = s[:n//2]
    b = s[n//2:]
    freq_a = Counter(a)
    freq_b = Counter(b)
    changes = 0
    for char in freq_a:
        if freq_a[char] > freq_b.get(char, 0):
            changes += freq_a[char] - freq_b.get(char, 0)
    return changes