pat乙级-1007 素数对猜想 (20)

#include <cstdlib>
#include <cmath>
#define N 100000
using namespace std;

int main()
{
    int n,i,j,a[N],k=1,counter=0,p=0;
    cin>>n;
    a[0]=1;
    for(i=3; i<=n; ++i)
    {
        for(j=2; j<=sqrt(i); ++j)
        {
            if(i%j==0)
                p++;
        }
        if(p==0)
        {
            a[k]=i;
            if(a[k]-a[k-1]==2)
            {
                counter++;
                a[k]=a[k-1];
            }
            k++;
        }

        p=0;
    }

    cout<<counter<<endl;
    return 0;
}