import sys import math import bisect from heapq import heapify, heappop, heappush from collections import deque, defaultdict, Counter from functools import lru_cache from itertools import accumulate, combinations, permutations, product sys.set_int_max_str_digits(10 ** 6) sys.setrecursionlimit(1000000) MOD = 10 ** 9 + 7 MOD99 = 998244353 input = lambda: sys.stdin.readline().strip() NI = lambda: int(input()) NMI = lambda: map(int, input().split()) NLI = lambda: list(NMI()) SI = lambda: input() SMI = lambda: input().split() SLI = lambda: list(SMI()) EI = lambda m: [NLI() for _ in range(m)] def main(): N = NI() H = NLI() INF = 10**18 D = [[INF, 1]] ans = 0 for i, h in enumerate(H): while D[-1][0] <= h: x, c = D.pop() if c == 0: ans -= x else: ans += x if D[-1][1] != i%2: if i%2 == 0: ans += h else: ans -= h D.append([h, i%2]) print(ans) if __name__ == "__main__": main()