#include <iostream>
#include <string>
using namespace std;

int main() {
    string S;
    cin >> S;
    int N = S.length();

    int flips = 0;
    int balance = 0; // 1s - 0s

    for (int i = 0; i < N; ++i) {
        if (S[i] == '1') {
            balance++;
        } else {
            balance--;
        }

        // If at any point balance goes negative, fix it by flipping a 0
        if (balance < 0) {
            flips++;
            balance += 2; // flip the current 0 to 1
        }
    }

    cout << flips << endl;
    return 0;
}