#include <bits/stdc++.h>
using namespace std;
const int INF = 2147483647;
const int INF2 = 0x3f3f3f3f;
const long long INF64 = 1e18;
const double INFD = 1e30;
const double EPS = 1e-6;
const double PI = acos(-1);
const int MOD = 1e9 + 7;
template <typename T>
inline T read() {
  T X = 0, w = 0;
  char ch = 0;
  while (!isdigit(ch)) {
    w |= ch == '-';
    ch = getchar();
  }
  while (isdigit(ch)) X = (X << 3) + (X << 1) + (ch ^ 48), ch = getchar();
  return w ? -X : X;
}
const int MAXN = 200005;
int n, m;
int dr[4] = {1, 0, -1, 0};
int dc[4] = {0, 1, 0, -1};
int dr2[8] = {1, 1, 1, -1, -1, -1, 0, 0};
int dc2[8] = {1, 0, -1, 1, 0, -1, 1, -1};
int CASE = 1;
char A[MAXN];
char B[MAXN];
long long Lsum[MAXN][27], Rsum[MAXN][27];
int main() {
  scanf("%d", &n);
  scanf("%s", A + 1);
  scanf("%s", B + 1);
  for (int i = 1; i <= n; i++) {
    for (int j = 0; j < 26; j++) {
      Lsum[i][j] = Lsum[i - 1][j];
    }
    Lsum[i][B[i] - 'A'] += i;
  }
  for (int i = n; i >= 1; i--) {
    for (int j = 0; j < 26; j++) {
      Rsum[i][j] = Rsum[i + 1][j];
    }
    Rsum[i][B[i] - 'A'] += n - i + 1;
  }
  double tot = 0;
  for (int i = 1; i <= n; i++) {
    long long li = i, ri = n - i + 1;
    long long a = ri * Lsum[i - 1][A[i] - 'A'],
              b = li * Rsum[i + 1][A[i] - 'A'];
    tot += a;
    tot += b;
    if (A[i] == B[i]) {
      tot += li * ri;
    }
  }
  double base = 0;
  for (int i = 1; i <= n; i++) {
    base += (long long)i * i;
  }
  printf("%.9lf\n", tot / (double)base);
  return 0;
}
