#include <bits/stdc++.h>
using namespace std;
template <class T>
inline T bigmod(T p, T e, T M) {
  long long ret = 1;
  for (; e > 0; e >>= 1) {
    if (e & 1) ret = (ret * p) % M;
    p = (p * p) % M;
  }
  return (T)ret;
}
template <class T>
inline T gcd(T a, T b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
template <class T>
inline T modinverse(T a, T M) {
  return bigmod(a, M - 2, M);
}
string s1, s2;
vector<int> vc1[27], vc2[27];
int main() {
  long long a, b = 0, c, d = 0, e, f = 0, g, h = 0, x, y, z = 0;
  cin >> a;
  cin >> s1 >> s2;
  for (int i = (0); i < (s1.size()); ++i) {
    vc1[s1[i] - 'A'].push_back(i);
    vc2[s2[i] - 'A'].push_back(i);
  }
  double p = 0, res = 0;
  for (int i = (1); i < (a + 1); ++i) p += (double)i * (double)i;
  for (int i = (0); i < (26); ++i) {
    double lft = 0.0, rgt = 0.0;
    for (int j = (0); j < (vc2[i].size()); ++j) {
      rgt += (double)(a - vc2[i][j]);
    }
    c = 0;
    for (int j = 0; j < vc1[i].size(); j++) {
      while (c < vc2[i].size() && vc2[i][c] < vc1[i][j]) {
        lft += (double)(vc2[i][c] + 1);
        rgt -= (double)(a - vc2[i][c]);
        c++;
      }
      res +=
          (lft * (double)(a - vc1[i][j]) + rgt * (double)(vc1[i][j] + 1)) / p;
    }
  }
  cout << fixed << setprecision(8) << res << endl;
}
