#include <bits/stdc++.h>
using namespace std;
clock_t time_p = clock();
void aryanc403() {
  time_p = clock() - time_p;
  cerr << "Time Taken : " << (float)(time_p) / CLOCKS_PER_SEC << "\n";
}
const long long int INF = 0xFFFFFFFFFFFFFFFL;
long long int seed;
mt19937 rng(seed = chrono::steady_clock::now().time_since_epoch().count());
inline long long int rnd(long long int l = 0, long long int r = INF) {
  return uniform_int_distribution<long long int>(l, r)(rng);
}
class CMP {
 public:
  bool operator()(pair<long long int, long long int> a,
                  pair<long long int, long long int> b) {
    return !(a.first < b.first || a.first == b.first && a.second <= b.second);
  }
};
void add(map<long long int, long long int> &m, long long int x,
         long long int cnt = 1) {
  auto jt = m.find(x);
  if (jt == m.end())
    m.insert({x, cnt});
  else
    jt->second += cnt;
}
void del(map<long long int, long long int> &m, long long int x,
         long long int cnt = 1) {
  auto jt = m.find(x);
  if (jt->second <= cnt)
    m.erase(jt);
  else
    jt->second -= cnt;
}
bool cmp(const pair<long long int, long long int> &a,
         const pair<long long int, long long int> &b) {
  return a.first < b.first || (a.first == b.first && a.second < b.second);
}
const long long int mod = 1000000007L;
const long long int maxN = 10000L;
long long int T, n, i, j, k, in, cnt, l, r, u, v, x, y;
long long int m;
string s;
map<long long int, long long int> cur;
map<long long int, vector<pair<long long int, long long int> > > a;
int main(void) {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  {
    long long int d;
    cin >> d >> n >> m;
    cur[INF] = 1;
    cur[0] = 1;
    a[n + 1].push_back({0, -1});
    a[d].push_back({INF, 1});
    while (m--) {
      cin >> x >> in;
      a[x + 1].push_back({in, 1});
      a[x + n + 1].push_back({in, -1});
    }
    long long int ans = 0, pvr = 1;
    for (const auto &x : a) {
      if (cur.begin()->first == INF) {
        cout << -1 << "\n";
        return 0;
      };
      ans += (min(x.first, d + 1) - pvr) * cur.begin()->first;
      if (x.first > d) break;
      for (auto y : x.second) {
        if (y.second > 0)
          add(cur, y.first);
        else
          del(cur, y.first);
      }
      pvr = x.first;
    }
    cout << ans << "\n";
  }
  aryanc403();
  return 0;
}
