#include <bits/stdc++.h>
using namespace std;
typedef signed long long ll;

#undef _P
#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define ITR(x,c) for(__typeof(c.begin()) x=c.begin();x!=c.end();x++)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
//-------------------------------------------------------

int N;
double R;

void solve() {
	int i,j,k,l,r,x,y; string s;
	
	cin>>N;
	R=1;
	for(i=2;i<=N;i++) {
		R+=2.0/i;
		if(i>2) {
			double tot=((i-1)*i/2)*((i-1)*i/2)-(i-1)*i*(2*i-1)/6;
			tot/=(i-1)*(i-2);
			R+=tot*(i-2.0)/i;
		}
	}
	_P("%.12lf\n",R);
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false);
	FOR(i,argc-1) s+=argv[i+1],s+='\n';
	FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	solve(); return 0;
}