#include<bits/stdc++.h>
using namespace std;
int nrprim(int x)
{
if(x<=1)
return 0;
if(x%2==0&&x!=2)
return 0;
for(int d=3;d*d<=x;d+=2)
if(x%d==0)
return 0;
return 1;
}
int main()
{
unsigned long long x,n,i,k=0,ap=0;
double q;
cin>>n;
for(i=1;i<=n;i++)
{
cin>>x;
if(nrprim(x))
{
k=k+x;
ap++;
}
}
q=(float)k/ap;
cout<<q-0.01;
return 0;
}