import io
(n,t,k) = input().split()
n = int(n)
t = int(t)
k = int(k)
c = []
for i in range(3,3+k):
c.append(int(input()))
#print(n,t,k)
l = int(c[0])
r = int(20*1e38)
while l < r:
m = (l+r)//2
s = 0
for i in range(0, k):
if m < c[i]:
break
dist = (m-c[i])//t + 1
s += 2*dist*(dist+1)
if s >= n :
r = m
else :
l = m+1
# buy everything that is "less" than
cum = 0
ans = 0
for i in range(0, k):
if l-1 < c[i]:
break
dist = (l-1-c[i])//t + 1
cum += 2*dist*(dist+1)
ans += 2*(c[i]-t)*dist*(dist+1)
ans += t*dist*(dist+1)*(2*dist+1)//3*2
ans += l*(n-cum)
print(ans)
