#include <iostream>
#include <cmath>

using namespace std;
int main() {
    string inputString;
    cin >> inputString;

    int x = 0;
    int maxZeros = 0;

    for ( int position = 1; position < inputString.size(); position++ ) {
        if( inputString[position] == '1' ) {
            int result = position-x-1;
            if ( maxZeros < result ) maxZeros = result;
            x = position;
        }
    }

    cout << ceil( maxZeros/2.0 );
    return 0;
}