// #pragma GCC optimize("Ofast")
// #pragma GCC optimize("unroll-loops")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2")
#include <bits/stdc++.h>
#define int long long
#define i64 long long
#define pii pair <int, int> 
using namespace std;
inline int read(void) {
    int x=0,sgn=1; char ch=getchar();
    while(ch<48||57<ch) {if(ch==45)sgn=0;ch=getchar();}
    while(47<ch&&ch<58) {x=x*10+ch-48;   ch=getchar();}
    return sgn? x:-x;
}
void write(int x) {
    if(x<0) putchar('-'),x=-x;
    if(x>9) write(x/10);
    putchar(x%10+'0');
}
/*
    write((Ans%p+p)%p); pls
*/
#define double long double
int n,sum;
int v[100005];
double r[100005];
double Ans;
signed main() {
    // freopen("localinput","r",stdin);
    // freopen("localoutput","w",stdout);
    n=read();
    for(int i=1; i<=n; ++i) sum+=(v[i]=read());
    for(int i=1; i<=n; ++i)
        for(int u=i<<1; u<=n; u+=i) 
            v[u]+=v[i];
    for(int i=1; i<=n; ++i) r[i]=1.0*sum/(sum-v[i]);
    for(int i=1; i<=n; ++i)
        for(int u=i<<1; u<=n; u+=i) 
            r[u]-=r[i];
    for(int i=1; i<=n; ++i) Ans+=r[i];
    printf("%.10Lf\n",Ans-1.0);
    // fprintf(stderr,"%.4lf\n",1.0*clock()/CLOCKS_PER_SEC);
    return 0;
}