#include <cmath>
#include <iostream>
using namespace std;
int main() {
	long long n;
	cin >> n, n = abs(n);
	int s = 0, l = n ? log10(n) : 1;
	do
		s += n % 10 * (l-- % 2);
	while(n /= 10);
	cout << s << endl;
}