N, P = map(int, input().split())
H = list(map(int, input().split()))
L = [H[0]]
pre = H[0]
for i in range(N):
    if H[i] == pre:
        continue
    L.append(H[i])
    pre = H[i]

N = len(L)
L.append(-10**18)
ans = 0
for i in range(1, N):
    ans += max(0, min(P, L[i] - L[i - 1], L[i] - L[i + 1]))
print(ans)