n = int(input())
s = "1" + input()

cnt = 0
z = 0
for c in range(n):
    if s[c] == "0":
        z += 1
    if s[c] == "1":
        if z % 2 == 0:
            cnt += z // 2
        else:
            cnt += z // 2 + 1

print(cnt)