#include <iostream>

using namespace std;
int main() {
    string inp;
    cin >> inp;
    char front = inp[0];
    char back = inp[1];
    int out= 0;
    bool flag = true;
    for (int i = 2; i < inp.length(); i = i+2) {
        char lstBack = back;
        char lstFront = front;
        front = inp[i];
        back = inp[i+1];
        if(lstFront != front){
            if(flag){
                out++;
            }
            flag = !flag;
        }
    }
    cout << out<< endl;
    return 0;
}