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TB_SNT_NhoHonK.cpp

#include <stdio.h>

int check(int n) {
	if(n < 2) return 0; // n not is prime
	for(int i = 2; i < n; i++) {
		if(n % i == 0) return 0; // n not is prime
	}
	return 1; // n is prime
}

int main() {
	int n;
	scanf("%d", &n);

	int count = 0, s = 0;
	for (int i = 2; i <= n; i++) {
		if( check(i) ) {
			count++;
			s += i;
		}
	} 
	printf("%.3f", (float)s / count);

	return 0;
}