AC

#include<bits/stdc++.h>
using namespace std;
int main()
{
int n,cnt=0,min=0,m=0;
cin>>n;
string s;
cin>>s;
int left=0,right=n-1;
while(left<right)
{
int l=left,r=right;
while(s[l]!=s[r]&&l<=r)r--;
if(lr)
{
min++;
if(n%20)
{
cout<<"Impossible";
return 0;
}
else
{
if(min>1)
{
cout<<"Impossible";
return 0;
}
m=l;
right++;
}

    }
    else if(r!=right)
    {
        for(int i=r;i<right;i++)
        {
            swap(s[i],s[i+1]);
            cnt++;
        }
    }
    left++;
    right--;
}
if(m)
    cout<<cnt+n/2-m;
else cout<<cnt;
return 0;

}链接描述