#include<stdio.h>
#include<string.h>
#include<math.h>
int main()
{
	int i = 0, k = 0, len = 0, ans1 = 0, ans2 = 0;//ans1 局部最优解  ans2全局最优解 
	char s[100005] = {0};
	scanf("%s", s+1);
	len = strlen(s+1);
	for(i = 1; i <= len; i++)
	{
		k = s[i] - '0';
		if(k == 0)
		{
			ans1++;
			ans2 = fmax(ans1, ans2);
		}
		else
		    ans1 = fmax(0, ans1-1);
	}
	printf("%d\n",ans2);
	return 0;
}