Function to compute all primes using Sieve of Eratosthenes
void sieve ( ) { prime [ 0 ] = prime [ 1 ] = false ; for ( int i = 2 ; i < 100005 ; i ++ ) prime [ i ] = true ; for ( int i = 2 ; i * i < 100005 ; i ++ ) { if ( prime [ i ] ) { for ( int j = i * i ; j < 100005 ; j += i ) { prime [ j ] = false ; } } } }