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

int minOperationsToGoodString(const string& S) {
    int balance = 0;
    int flips = 0;
    
    for (char ch : S) {
        if (ch == '1') {
            balance++;
        } else {
            balance--;
        }

        // If balance goes negative, flip a 0 to a 1
        if (balance < 0) {
            flips++;
            balance += 2; // simulate flipping a 0 to a 1
        }
    }

    return flips;
}

int main() {
    string S;
    cin >> S;
    cout << minOperationsToGoodString(S) << endl;
    return 0;
}