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

balance = 0
ans = 0

for c in s:
    if c == '1':
        balance += 1
    else:  # c == '0'
        balance -= 1
    
    if balance < 0:
        ans += 1
        balance += 2  # この0を1に変えたと仮定してbalance補正

print(ans)