N = int(input())
A = [0] + list(map(int, input().split())) + [10**18]


ans = 0
for i in range(1, N + 1):
    if A[i] <= A[i + 1]:
        continue
    # 左をできるだけ下げる
    d = A[i] - max(A[i - 1], A[i + 1])
    ans += d
    A[i] -= d
    if A[i] <= A[i + 1]:
        continue
    # 右を上げる
    d = A[i] - A[i + 1]
    ans += d
    A[i+1] += d

print(ans)