696 B | 696 chars
import sys
from math import sqrt, pi, sin, factorial
inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = " " : [*map(int,inp().split(x))]
mfi = lambda x = " " : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def solve():
E, L, B = mii()
l = [ii() for _ in range(B)]
INF = 1111111111111111111
D = [INF] * 44000
chk = [0] * 44000
for i in l:
chk[i] = 1
D[0] = 0
for i in range(1, E + 1):
if chk[i]:
continue
for j in range(1, L + 1):
D[i] = min(D[i - j] + 1, D[i])
print(D[E])
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
solve()