Codeforces Beta Round #50 A. Presents
N,K = map(int,raw_input().split()) C = [0]+map(int,raw_input().split())[1:]+[N+1] print sum([1+(C[i]-C[i-1]-1)/K for i in range(1,len(C))])-1
N,K = map(int,raw_input().split()) C = [0]+map(int,raw_input().split())[1:]+[N+1] print sum([1+(C[i]-C[i-1]-1)/K for i in range(1,len(C))])-1