def min_operations_to_make_good(s):
    n = len(s)
    s_list = list(s)
    operations = 0
    
    for i in range(n):
        zeros = ones = 0
        zero_positions = []
        
        for j in range(i, n):
            if s_list[j] == '0':
                zeros += 1
                zero_positions.append(j)
            else:
                ones += 1
                
            if zeros > ones:
                if zero_positions:
                    leftmost_zero_idx = zero_positions[0]
                    s_list[leftmost_zero_idx] = '1'
                    operations += 1
                    zeros -= 1
                    ones += 1
                    zero_positions.pop(0)
    
    return operations

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