N = int(input())
S = list(input())
cnt = 0
ans = 0
L = [0, 0, 1]
for i in range(N):
    if(S[i] == "0"):
        cnt += 1
    else:
        ans += (cnt // 3) * 2 + L[cnt % 3]
        cnt = 0
print(ans + (cnt // 3) * 2 + L[cnt % 3])