import sys input = lambda :sys.stdin.readline()[:-1] ni = lambda :int(input()) na = lambda :list(map(int,input().split())) yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES") no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO") ####################################################################### n = ni() h = [10 ** 18] + na() ans = 0 sk = [0] for i in range(1, n + 1): while sk and h[sk[-1]] <= h[i]: x = sk.pop() ans += h[x] * (sk[-1] % 2 - x % 2) # print(sk, ans) ans += h[i] * (i % 2 - sk[-1] % 2) sk.append(i) print(ans)