import numpy as np

def min_operations_to_make_good(s):
    n = len(s)
    arr = np.array([int(c) for c in s])
    changed_positions = set()
    
    for i in range(n-1):
        zeros = 0
        ones = 0
        
        for j in range(i, n):
            if arr[j] == 0:
                zeros += 1
            else:
                ones += 1
                
            if j > i and zeros >= ones:
                for k in range(i, j+1):
                    if arr[k] == 0:
                        arr[k] = 1
                        changed_positions.add(k)
                        zeros -= 1
                        ones += 1
                        break
                        
    return len(changed_positions)

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