#include 
using namespace std;

int sum(int n) {
    if(n <= 0) {
        return 0;
    }
    
    return sum(n / 10) + (n % 10);
}
int main() {
    cout << sum(123) << endl;
    return 0;
}

// sum(123) = sum(12) + 3 = 3 + 3 = 6
// sum(12) = sum(1) + 2 = 1 + 2 = 3
// sum(1) = sum(0) + 1 = 0 + 1 = 1
// sum(0) = 0