def min_operations_to_make_good(s):
    n = len(s)
    arr = [int(c) for c in s]
    changed = set()
    
    for i in range(n-1):
        balance = 0
        zero_positions = []
        
        for j in range(i, n):
            if arr[j] == 0:
                balance -= 1
                zero_positions.append(j)
            else:
                balance += 1
            
            if j > i and balance <= 0 and zero_positions:
                pos = zero_positions[0]
                zero_positions.pop(0)
                if pos not in changed:
                    arr[pos] = 1
                    changed.add(pos)
                    balance += 2
    
    return len(changed)

n = int(input())
s = input().strip()
print(min_operations_to_make_good(s))