590 B | 590 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():
t, s , n = mii()
l = mii()
fall = s
prev = 0
for i in l:
time = i - prev
fall += min(time, s)
fall = min(fall, s)
fall = s - fall
prev = i
k = max(0, s - fall - (t - prev))
print(k)
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
solve()