public class Solution {

```
public int CountPrimes(int n) {
bool[] isPrime = new bool[n];
//Initialize
for( int i = 2; i < n; i++){
isPrime[i] = true;
}
// mark-off
for(int i = 2; i*i < n; i++){
if( !isPrime[i]) continue;
for( int j = i*i; j<n; j += i){
isPrime[j] = false;
}
}
// count
int count = 0;
for(int i = 2; i< n; i++){
if( isPrime[i]) count++;
}
return count;
}
```

}