"""
[25630: 팰린드롬 소떡소떡](https://www.acmicpc.net/problem/25630)
Tier: Bronze 3
Category: 구현
"""
def solution(s):
cnt = 0
for i in range(len(s) // 2):
if s[i] != s[-(i+1)]:
cnt += 1
return cnt
if __name__ == '__main__':
input()
print(solution(input()))