#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  long long int t = 1;
  while (t--) {
    long long int k, b, n;
    cin >> k >> b >> n;
    long long int s = 0;
    long long int a[n];
    map<long long int, vector<long long int> > mp;
    mp[0].push_back(-1);
    for (long long int i = 0; i < n; i++) {
      cin >> a[i];
      s = s + a[i];
      s = s % (k - 1);
      mp[s].push_back(i);
    }
    long long int an = 0;
    if (b == k - 1 || b == 0) {
      map<long long int, long long int> mp;
      mp[0]++;
      long long int an = 0, z = 0, ts = 0;
      for (long long int i = 0; i < n; i++) {
        ts = (ts + a[i]) % (k - 1);
        if (a[i] == 0)
          z++;
        else
          z = 0;
        long long int tmp = (ts + (k - 1) - b) % (k - 1);
        if (b == 0)
          an += z;
        else
          an = an + mp[tmp] - z;
        mp[ts]++;
      }
      cout << an;
    } else {
      for (auto x : mp) {
        long long int y = x.first;
        y = (y + b) % (k - 1);
        for (long long int i = 0; i < x.second.size(); i++) {
          long long int l =
              lower_bound(mp[y].begin(), mp[y].end(), x.second[i]) -
              mp[y].begin();
          an = an + max(0ll, (long long int)(mp[y].size() - l));
        }
      }
      cout << an;
    }
  }
  return 0;
}
