Codeforces Round #498 (Div. 3), problem: (D) Two Strings Swaps

#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization ("unroll-loops")

#include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define pii pair<int,int>
#define pli pair<long long int,int>
#define pbi pair<bool,int>
#define pob pop_back
#define all(a) a.begin(),a.end()
#define fio ios_base::sync_with_stdio(false); cin.tie(0);cout.tie(0);
#define i64 long long int
#define mem(x,y) memset(x,y,sizeof(x))
#define fill(arr,b) fill(arr, arr+sizeof(arr)/sizeof(arr[0]), b)
#define maxarr(n,arr,m) m =-1e9;for(int i=0;i<n;i++)if(arr[i]>m) m=arr[i];
#define read freopen("input.txt","r",stdin);

const double pi = acos(-1.0);
const int maxn = (i64)2e9+5;

int main()
{
    int n;
    string a,b;
    cin >> n;
    cin >> a >> b;
    int ans = 0;

    for(int i = 0, j = n-1 ; i<=j ; i++,j--)
    {
        if(i==j && a[i]!=b[i])ans++;
        else if(a[i]==b[i]&&a[j]==b[j])continue;
        else if(a[i]==b[i]||a[j]==b[j])ans++;
        else if(a[i]==b[j]&&a[j]==b[i]) continue;
        else if(a[i]==a[j]&&b[i]==b[j]) continue;
        else
        {
            set<char>s{a[i],a[j],b[i],b[j]};
            if(s.size()==4) ans+=2;
            else
            {
                ans++;
                if(a[i]==a[j]) ans++;
            }

        }
    }
    cout << ans;
    return 0;
}