#include <algorithm>
#include <cmath>
#include <cstdio>
#include <iostream>
#include <string>

using namespace std;

const double pi = acos(-1.0);

int n;
string s;

double Dist(double x1, double y1, double x2, double y2) {
  double dx = x2 - x1;
  double dy = y2 - y1;
  return sqrt(dx * dx + dy * dy);
}

double Adjust() {
  int i = 0;
  while (i < n && s[i] == 'T') ++i;
  if (i == n) return -0.5;
  if (i == 0) {
    if (s[i] == 'S') return 0;
    else return -2 + pi / 2;
  }
  double x1 = 0.5;
  double y1 = sqrt(3) / 2;
  if (s[i] == 'S') {
    double x2 = i;
    double y2 = 1.0;
    return -x2 - 1 + Dist(0, 0, x1, y1) + Dist(x1, y1, x2, y2); 
  } else {
    double xc = i + 0.5;
    double yc = 0.5;
    double h = Dist(x1, y1, xc, yc);
    double alpha = atan2(y1 - yc, x1 - xc);
    double fi = acos(0.5 / h);
    double x2 = xc + 0.5 * cos(alpha - fi);
    double y2 = yc + 0.5 * sin(alpha - fi);
    return -xc - 1 + Dist(0, 0, x1, y1) + Dist(x1, y1, x2, y2) +
           0.5 * (alpha - fi - pi / 2);
  }
}

int main() {
  cin >> n >> s;

  double ret = 2 * n + 2;
  ret += Adjust();
  reverse(s.begin(), s.end());
  ret += Adjust();
  printf("%.6lf\n", ret);
  
  return 0;
}