#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int count0 = 0;
  int count1 = 0;
  for (char i : s) {
    if (i == '0')
      count0++;
    else
      count1++;
  }
  cout << count0 << endl;
  return 0;
}
