#include<bits/stdc++.h>
#define CSP2026 using
#define RP namespace
#define PLUSPLUS std
CSP2026 RP PLUSPLUS;
#define debug(x) cout<<"clock="<<clock()<<' '<<#x<<'='<<(x)<<'\n'
//#define getchar getchar_unlocked
using ll=long long;
inline int rd(){int x=0,f=1;char ch=getchar();for(;!isdigit(ch);ch=getchar())if(ch=='-')f=-1;for(;isdigit(ch);ch=getchar())x=(x<<3)+(x<<1)+(ch^48);return x*f;}
inline ll rdll(){ll x=0;int f=1;char ch=getchar();for(;!isdigit(ch);ch=getchar())if(ch=='-')f=-1;for(;isdigit(ch);ch=getchar())x=(x<<3)+(x<<1)+(ch^48);return x*f;}
signed main(){
	string s;cin>>s;
	int l=s.size(),ans=0;
	for(int i=1;i<=l/2;++i)if(s[i-1]!=s[l-i])++ans;
	cout<<ans<<'\n';
}