import std.stdio;
import std.algorithm;
import std.string;
import std.range;
import std.array;
import std.conv;
import std.complex;
import std.math;
import std.ascii;
import std.bigint;
import std.container;
import std.typecons;

auto readInts() {
	return array(map!(to!int)(readln().strip().split()));
}
auto readInt() {
	return readInts()[0];
}
auto readLongs() {
	return array(map!(to!long)(readln().strip().split()));
}
auto readLong() {
	return readLongs()[0];
}

const real eps = 1e-10;

void main(){
	auto notPrime = new bool[](123456*2+1);
	notPrime[0] = notPrime[1] = true;
	for(int i; i < notPrime.length; ++i) {
		if(!notPrime[i]) {
			for(int j = 2; i*j < notPrime.length; ++j) {
				notPrime[i*j] = true;
			}
		}
	}

	while(true) {
		auto n = readInt();
		if(n == 0) {
			return;
		}
		int ans;
		for(int i = n+1; i <= 2*n; ++i) {
			if(!notPrime[i]) {
				++ans;
			}
		}
		writeln(ans);
	}
}