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- #include <fstream>
- #include <vector>
- using namespace std;
- vector<bool>prim(50000001,true);
- void Ciur(int n)
- {
- prim[0]=prim[1]=false;
- for(int j=2;j*j<=n;++j)
- if(prim[j])
- for(int i=j*2;i<=n;i+=j)
- prim[i]=false;
- }
- int main()
- {
- int n;
- ifstream f("primcolor.in");
- f>>n;
- f.close();
- ofstream g("primcolor.out");
- if(n<=3)
- g<<n;
- else
- {
- Ciur(n);
- int sol=2;
- for(int i=n/2+1;i<=n;++i)
- if(prim[i])
- ++sol;
- g<<sol;
- }
- g.close();
- return 0;
- }
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