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- #include <fstream>
- #include<bitset>
- using namespace std;
- int main()
- {
- bitset<500000>fr;
- fr.reset();
- for(int i=1;i<500000;++i)
- if(fr[i]==0)
- for(int j=3*i+1;j<500000;j+=(i<<1)+1)
- fr[j]=1;
- int x,y,nr=0;
- ifstream f("eratostene1.in");
- f>>x;
- ++x;
- fr[0]=1;
- while(--x)
- {
- f>>y;
- if(y==2)
- ++y;
- if(y&1)
- nr+=1-fr[(y>>1)];
- }
- f.close();
- ofstream g("eratostene1.out");
- g<<nr;
- g.close();
- return 0;
- }
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