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Cod sursa(job #790624)
#include <cstdio>
#define maxN 2000005
using namespace std;
FILE *f = fopen ("ciur.in","r");
FILE *g = fopen ("ciur.out","w");
int n, sol;
bool prim[maxN];
int main()
{
fscanf (f, "%d", &n);
for (int i = 2; i <= n; i++)
prim[i] = true;
for (int i = 2; i <= n; i++)
if (prim[i]) {
sol++;
for (int j = 2 * i; j <= n; j += i)
prim[j] = 0;
}
fprintf (g, "%d\n", sol);
fclose(f);
fclose(g);
return 0;
}