This won't take much time

#include<bits/stdc++.h>
using namespace std;
bool alpha[123456790];
void solve(){
    memset(alpha,1,sizeof alpha);
    alpha[0]=alpha[1]=0;
    int ans=0;
    for(int i=2;i<123456790;i++){
        if(alpha[i]){
            ans++;
            if(1LL*i*i<123456790){
                int j=i*i;
                while(j<123456790){
                    alpha[j]=false;
                    j+=i;
                 }
            }
        }
     }
    cout<<ans;
}