#include <bits/stdc++.h>   using namespace std;   template <class T>   inline void chkmin(T &a, T b) {     if (a > b) a = b;   }   template <class T>   inline void chkmax(T &a, T b) {     if (a < b) a = b;   }   long long sum[30];   char fir[210000], sec[210000];   int main() {     int len, i;     double ans, cnt;     scanf( %d , &len);     scanf( %s%s , fir, sec);     cnt = 1LL * len * (len + 1) * (2 * len + 1) / 6;     ans = 0;     memset(sum, 0, sizeof(sum));     for (i = len - 1; i >= 0; i--) {       sum[fir[i] -  A ] += len - i;       ans += sum[sec[i] -  A ] * (i + 1) / cnt;     }     memset(sum, 0, sizeof(sum));     for (i = len - 1; i >= 0; i--) {       ans += sum[fir[i] -  A ] * (i + 1) / cnt;       sum[sec[i] -  A ] += len - i;     }     printf( %.15lf n , 1.0 * ans);     return 0;   }   