import sys

# sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = 18446744073709551615
# inf = 4294967295
# md = 10**9+7
md = 998244353

def solve(p):
    bb = [a if i & 1 == p else 1-a for i, a in enumerate(aa)]
    bb[-1] = 0
    for i in range(n-1)[::-1]:
        if bb[i]: bb[i] += bb[i+1]
    res = 0
    # print(p, bb)
    for i, b in enumerate(bb):
        if i & 1 == p: res += b
    return res

n = II()
aa = LI()
ans = solve(0)+solve(1)
print(ans)