import java.io.IOException; import java.io.InputStream; import java.util.NoSuchElementException; public class Main { public static void main(String[] args) { FastScanner sc = new FastScanner(); double x = Double.parseDouble(sc.next()); double org = Math.PI * Math.PI / 6; double a = 0; double b = 0; for (double n = 1; n < 10000000; n ++) { a += 1.0 / ((x + n) * (x + n)); b += 1.0 / (n * n); } a += (org - b); System.out.printf("%.15f%n", a); } } class FastScanner { public static String debug = null; private final InputStream in = System.in; private int ptr = 0; private int buflen = 0; private byte[] buffer = new byte[1024]; private boolean eos = false; private boolean hasNextByte() { if (ptr < buflen) { return true; } else { ptr = 0; try { if (debug != null) { buflen = debug.length(); buffer = debug.getBytes(); debug = ""; eos = true; } else { buflen = in.read(buffer); } } catch (IOException e) { e.printStackTrace(); } if (buflen < 0) { eos = true; return false; } else if (buflen == 0) { return false; } } return true; } private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1; } private static boolean isPrintableChar(int c) { return 33 <= c && c <= 126; } private void skipUnprintable() { while (hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; } public boolean isEOS() { return this.eos; } public boolean hasNext() { skipUnprintable(); return hasNextByte(); } public String next() { if (!hasNext()) throw new NoSuchElementException(); StringBuilder sb = new StringBuilder(); int b = readByte(); while (isPrintableChar(b)) { sb.appendCodePoint(b); b = readByte(); } return sb.toString(); } public long nextLong() { if (!hasNext()) throw new NoSuchElementException(); long n = 0; boolean minus = false; int b = readByte(); if (b == '-') { minus = true; b = readByte(); } if (b < '0' || '9' < b) { throw new NumberFormatException(); } while (true) { if ('0' <= b && b <= '9') { n *= 10; n += b - '0'; } else if (b == -1 || !isPrintableChar(b)) { return minus ? -n : n; } else { throw new NumberFormatException(); } b = readByte(); } } public int nextInt() { return (int) nextLong(); } public long[] nextLongList(int n) { return nextLongTable(1, n)[0]; } public int[] nextIntList(int n) { return nextIntTable(1, n)[0]; } public long[][] nextLongTable(int n, int m) { long[][] ret = new long[n][m]; for (int i = 0; i < n; i ++) { for (int j = 0; j < m; j ++) { ret[i][j] = nextLong(); } } return ret; } public int[][] nextIntTable(int n, int m) { int[][] ret = new int[n][m]; for (int i = 0; i < n; i ++) { for (int j = 0; j < m; j ++) { ret[i][j] = nextInt(); } } return ret; } }