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OI-codes/Luogu/P3912/P3912.cpp

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#include <bits/stdc++.h>
using namespace std;
int main() {
int n;
cin >> n;
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int s = n - 1;
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bool prime[100000005];
memset(prime, 0, sizeof(prime));
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for (int i = 2; i * i <= n; i++) {
if (!prime[i]) {
for (int j = i * 2; j <= n; j += i) {
if (!prime[j]) {
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prime[j] = true;
s--;
}
}
}
}
cout << s << endl;
return 0;
}