题目

#include<bits/stdc++.h>
using namespace std;
char s[200005];
int num1,num2,numall;
int main()
{
//	freopen("lock.in","r",stdin);
//	freopen("lock.out","w",stdout);
	string a;
	cin>>a;
	numall=a.length();
	for(int i = 1 ;i <=a.length();i++)
	{
		 s[i]=a[i-1];
		 if(a[i-1]=='a') num1++;
		 if(a[i-1]=='b') num2++;
	}
    if(num1%2==1&&num2%2==1)  { cout<<-1 ;  return 0; }
    else
    {
    
    		int ans=0;
    		for(int i=1;i<=numall/2;i++)
    		{
    			//cout<<s[i]<<" "<<s[numall-i]<<endl;
    			if(s[i]!=s[numall-i+1]) ans++;
    		}
		   if(ans%2==0) {cout<<ans/2;return 0;}
			else{cout<<ans/2+1;return 0;}
    	
    	
    }
    
	return 0;
}
// a b a b a