#include <stdio.h>
int main() {
    int arr[] = {1,3,5,7,9};
int l = 0, r = 4, x = 6, ans = -1;
while (l <= r) {
    int m = (l + r) / 2;
    if (arr[m] < x) l = m + 1;
    else if (arr[m] > x) r = m - 1;
    else { ans = m; break; }
}
printf("%d\n", ans);
    
    return 0;
}