var a:Character = " "
var b:Character = " "
var ct = 0
for c in s {
 if a=="P" && b=="O" && c=="E" {
  ct += 1
 }
 a = b
 b = c
}
return ct


Normally, to negate, you just put a minus sign to the left of the thing you want to negate.

y = -x  sets to the negation of x

y = x - 2*x

y = 0 - x

tf2!.text = String (ct)