binary_search

#include <iostream>

using namespace std;

int a[100], x;

int bin_search( int L, int R ){
    int k = (L+R)/2;
//  if(( x > a[k] && x > a[R] ) || ( x < a[k] && x < a[L])) return -1; //stupid
    if( L > R ) return -1;
    if( x == a[k] ) return k;
    x > a[k] ? bin_search( k+1, R ) : bin_search( L, k-1 );
}

int main(){

    for( int i=0; i<10; i++ ) a[i] = 2*i;
    for( int i=0; i<10; i++ ) cout <<i << ": "<< a[i] << endl;
    while( cin >> x ){

    cout << bin_search( 0, 9 ) << endl;

    }
    return 0;
}