arduino programing

hi,everybody.
please can me write program following issue?

in fact want write program use parking lot.

 the input used p1 barrier @ car park entrance .
 and p2 input used barrier @ exit of parking lot
 led1 must indicate when there 5 cars in parking lot .
 led2 must indicate when there less 5 in parking lot .
 bellow how started not know how continue need help

#define led1 4
#define led2 5
#define led1on digitalwrite (led1,0)
#define led1off digitalwrite (led1,1)
#define led2on digitalwrite (led2,0)
#define led2off digitalwrite (led2,1)
#define p1 2
#define p2 3
#define p1on (digitalread (p1)==0)
#define p2on (digitalread (p2)==0)

void setup() {
  pinmode (p1,input);
  pinmode (p2,input);
  pinmode (led1,output);
  pinmode (led2,output);

}

void loop() {
  // want program put here:

}

how know how many cars/spaces there are?

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