#include <bits/stdc++.h>
using namespace std;
const int K = 26;
int cnt[K];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long n;
  cin >> n;
  if (n < 1e6) {
    int ans = -1;
    for (long long i = max(n / 2 - 1000000ll, 0ll); i <= n / 2 + 1000000; i++) {
      long long j = n - i;
      if (j < 0) break;
      int a = 0, b = 0;
      long long x = i, y = j;
      while (x > 0) {
        a += x % 10;
        x /= 10;
      }
      while (y > 0) {
        b += y % 10;
        y /= 10;
      }
      ans = max(ans, a + b);
    }
    cout << ans;
    return 0;
  }
  long long deg = 1;
  while (deg < n) {
    deg *= 10ll;
  }
  deg /= 10ll;
  long long x = (n / deg) * deg - 1ll;
  long long y = n - x;
  int a = 0, b = 0;
  while (x > 0) {
    a += x % 10;
    x /= 10;
  }
  while (y > 0) {
    b += y % 10;
    y /= 10;
  }
  int ans = a + b;
  cout << ans;
}
