h, w = map(int,input().split())
blocks = list(map(int, input().split()))
h_max = 0
# ๊ฐ์ฅ ํฐ ๋์ด๋ฅผ ๊ฐ์ง ๋ธ๋ญ์ ๊ตฌํจ
for i in range(len(blocks)):
if h_max < blocks[i]:
h_max = blocks[i]
h_idx = i
# ์ค๋ฅธ์ชฝ ๊ตฌ์ญ
res = 0
val = 0
for i in range(h_idx+1):
if val < blocks[i]:
val = blocks[i]
res += val
# ์ผ์ชฝ ๊ตฌ์ญ
val = 0
for i in range(w-1, h_idx, -1):
if val < blocks[i]:
val = blocks[i]
res += val
print(res-sum(blocks))