import numpy as np

def min_operations_to_make_good(s):
    n = len(s)
    s_array = np.array(list(s))
    operations = 0
    
    for i in range(n):
        zeros = ones = 0
        zero_indices = []
        
        for j in range(i, n):
            if s_array[j] == '0':
                zeros += 1
                zero_indices.append(j)
            else:
                ones += 1
            
            if j - i + 1 >= 2:
                if zeros >= ones and len(zero_indices) > 0:
                    leftmost_zero = zero_indices.pop(0)
                    s_array[leftmost_zero] = '1'
                    operations += 1
                    zeros -= 1
                    ones += 1
    
    return operations

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