import sys input = lambda: sys.stdin.readline().rstrip() def main(): # 入力 N = int(input()) H = list(map(int, input().split())) # 計算・出力 stack = [(10**10, -1)] # 色の変わり目と色 0:そこまで緑, 1:そこまで水 ans = 0 for i, h in enumerate(H): c = -1 if (i & 1) else 1 while stack[-1][0] <= h: ph, pc = stack.pop() ans -= ph * pc if stack[-1][1] != c: stack.append((h, c)) ans += h * c print(ans) # print(f'{stack = }') if __name__ == "__main__": main()