def min_operations(n, s):
    """
    For a good string, any substring must have count(0) <= count(1).
    
    Key insight: To ensure all substrings have this property, we need to ensure
    that each position's running balance (count(1) - count(0)) never goes below
    the minimum balance seen so far minus 1.
    
    We use a greedy approach: When we encounter a position that violates this rule,
    we need to convert enough 0s to 1s to fix the balance.
    """
    balance = 0  # Current balance: count(1) - count(0)
    operations = 0  # Total operations needed
    
    for char in s:
        # Update balance
        if char == '1':
            balance += 1
        else:  # char == '0'
            balance -= 1
        
        # If balance becomes negative, we need to convert some 0s to 1s
        if balance < 0:
            operations += abs(balance)  # Convert enough 0s to make balance 0
            balance = 0  # After conversion, balance becomes 0
    
    return operations

def main():
    n = int(input())
    s = input().strip()
    print(min_operations(n, s))

if __name__ == "__main__":
    main()