n = int(input())
s = input()

count = 0

news = s

state = 0

for i in range(n):
    if state == 0:
        if news[i] == '0':
            state = 1
    
    elif state == 1:
        if news[i] == '0':
            state = 2
            count += 1
        else:
            state = 2
    elif state == 2:
        if news[i] == '0':
            count += 1
            state = 0
        else:
            state = 0

print(count)