import std.stdio, std.conv, std.math, std.string, std.range;
void main(){
  auto N = readln().strip().to!int;
  auto rec = recurrence!("(19*a[n-1] - 12*a[n-2])/4.to!real")(4.0, 3.0);
  rec.popFrontN(N);
  writefln("%1.10f", rec.front());
}