text: >>
  t=int(input())
  g=int(input())
  n=int(input())
  
  f=[]
  flag,i=0,0
  
  e=list(map(float,input().split()))
  e.append(t)
  
  while(i<t):
      p=i
      i+=g
      if (i in e): f.append(i)
      else:
          for z in range(0,g):
              if (i in e):
                  f.append(i)
                  break
              else: i-=1
          if(p==i):
              flag=1
              break
              
  if (flag==0): print(len(f)-1)
  else: print(-1)
  
  
  
  @ CODED BY TSG405, 2021
  