package chelper;

import java.util.ArrayList;
import java.util.List;

import io.InputReader;
import io.OutputWriter;
import misc.GCJSolution;
import misc.SimpleSavingChelperSolution;


public class TaskA extends GCJSolution {

  public void solve(int testNumber, InputReader in, OutputWriter out) {
    wrapSolve(testNumber, in, out);
  }

  long get(List<Integer> a) {
    long res = 0;
    long t = 1;
    for (int i : a) {
      res += i * t;
      t *= 2;
    }
    return res;
  }

  @Override
  public void solve(int testNumber) {
    long d = in.nextLong();
    String s = in.nextString();

    List<Integer> a = new ArrayList<>();

    int t = 0;

    for (char c : s.toCharArray()) {
      if (c == 'C') {
        a.add(t);
        t = 0;
      }
      if (c == 'S') {
        t++;
      }
    }
    if (t > 0) {
      a.add(t);
    }

    int c = 0;

    while (get(a) > d) {
      c++;
      if (a.size() == 1) {
        out.println("IMPOSSIBLE");
        return;
      }

      a.set(a.size() - 1, a.get(a.size() - 1) - 1);
      a.set(a.size() - 2, a.get(a.size() - 2) + 1);

      if (a.get(a.size() - 1) == 0) {
        a.remove(a.size() - 1);
      }               
    }

    out.println(c);
  }
}