#include <stdio.h>
float jge(int m){
int sum = 0,count = 0;
int i, j;
if (m < 100)
return 0;
else
for (i=100;i<=m;i++){
int priber = 1;
for (j=2;j<=i/2;j++)
if (i % j == 0)
priber = 0;
if (priber == 1){
sum += i;
count ++;
}
}
return (float)sum/count;
}
int main(){
float result=0;
int m;
scanf("%d", &m);
result = jge(m);
printf("%.2f\n", result);
}