#include <bits/stdc++.h>
using namespace std;
const long long inf = 1e5;
long long n, f[30][30];
long long getdig(long long a, long long pos) {
  for (long long i = (long long)1; i <= (long long)pos - 1; ++i) a /= 10;
  return a % 10;
}
long long dp(long long pos, long long nho) {
  if (f[pos][nho] != -1) return f[pos][nho];
  if (pos == 0 && nho == 0)
    return 0;
  else if (pos == 0)
    return -inf;
  long long res = -inf;
  long long tmp = getdig(n, pos);
  for (long long i = (long long)0; i <= (long long)9; ++i)
    for (long long j = (long long)0; j <= (long long)9; ++j)
      for (long long nexnho = (long long)0; nexnho <= (long long)1; ++nexnho) {
        long long sum = i + j + nexnho;
        if (sum / 10 != nho) continue;
        if (sum % 10 != tmp) continue;
        res = max(res, i + j + dp(pos - 1, nexnho));
      }
  return f[pos][nho] = res;
}
int main() {
  cin >> n;
  memset(f, -1, sizeof(f));
  cout << dp(13, 0);
}
