#include <iostream>
#include <cstdlib>
int main(){
    int n;
    if(!(std :: cin >> n)
    {
        return 0;
    }
    if (n < 0)
    {
        std::cout <<
        "Invalid input"
        << std :: endl;
        return 0;
    }
    int sum = 0;
    int current_number = n;
    while (current_number > 0
    {
        sum +=current_number /= 10;
        current_number /= 10;
    }
    std:: cout<<sum<<std::endl;
    return 0;
}