Slap Merge

Will you merge many policemen to slap robbers or let the policemen fight alone？ 
When a large number of robbers come to rob famous paintings, you need to send out the police force to resist the attack of robbers;
They will come like a flood. You need to judge the situation and put our police force in the right position;
The position of the police can be moved by dragging. When two policemen of the same level touch each other, they will merge into higher-level policemen and can slap more robbers.