#include <bits/stdc++.h>
using namespace std;
template <typename G>
struct triple {
  G first, second, T;
};
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long d, n, m;
  cin >> d >> n >> m;
  vector<pair<long long, long long> > x(m + 2);
  x[m + 1].first = d;
  for (int i = 1; i <= m; i++) cin >> x[i].first >> x[i].second;
  sort(x.begin(), x.end());
  vector<int> mn(m + 2);
  stack<int> s;
  for (int i = m + 1; i >= 0; i--) {
    while (!s.empty() && x[s.top()].second > x[i].second) s.pop();
    if (!s.empty())
      mn[i] = s.top();
    else
      mn[i] = m + 2;
    s.push(i);
  }
  long long comb = n, ans = 0;
  for (int i = 0; i < m + 1; i++) {
    int p = mn[i];
    long long t = min(n - comb, max(x[p].first - x[i].first - comb, 0LL));
    ans += x[i].second * t;
    comb += t;
    comb -= x[i + 1].first - x[i].first;
    if (comb < 0) {
      cout << "-1\n";
      return 0;
    }
  }
  cout << ans << '\n';
  return 0;
}
