def min_operations_to_good_string(S):
    ones = S.count('1')
    operations = 0
    
    while True:
        i = 0
        found_bad_sequence = False
        
        while i < len(S):
            if S[i] == '0':
                start = i
                while i < len(S) and S[i] == '0':
                    i += 1
                length = i - start
                
                if length >= 2 and length >= ones:
                    found_bad_sequence = True
                    mid = start + length // 2
                    S = S[:mid] + '1' + S[mid+1:]
                    ones += 1
                    operations += 1
                    break
            else:
                i += 1
        
        if not found_bad_sequence:
            break
    
    return operations

if __name__ == "__main__":
    N = int(input())
    S = input().strip()
    print(min_operations_to_good_string(S))