If enough -1/-1 counters are put on a creature at the same time to make its toughness 0 or less, the number of +1/+1 counters on it before it got any -1/-1 counters will be used to determine how many counters you put on target creature. For example, if there are three +1/+1 counters on Reyhan and it gets six -1/-1 counters, the target creature gets three +1/+1 counters.