#include <bits/stdc++.h>   using namespace std;   const int NMAX = 200010, ALPHA = 30;   int N;   long long Sum[ALPHA];   double Ans;   char A[NMAX], B[NMAX];   int main() {     scanf( %i n , &N);     gets(A + 1);     gets(B + 1);     long long Posib = (1LL * N * (N + 1) * (2 * N + 1)) / 6;     for (int i = N; i; --i) {       int C = B[i] -  A  + 1;       Sum[C] += N - i + 1;       C = A[i] -  A  + 1;       Ans += (1LL * i * Sum[C]) / (1.0 * Posib);     }     for (int i = 0; i < ALPHA; ++i) Sum[i] = 0;     for (int i = N; i; --i) {       int C = B[i] -  A  + 1;       Ans += (1LL * i * Sum[C]) / (1.0 * Posib);       C = A[i] -  A  + 1;       Sum[C] += N - i + 1;     }     printf( %.9f n , 1.0 * Ans);   }   