#include <bits/stdc++.h>
using namespace std;
const int maxN = 200 * 1000 + 10;
pair<int, int> a[maxN];
int nx[maxN];
stack<int> curr;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int d, n, m;
  cin >> d >> n >> m;
  for (int i = 0; i < m; i++) cin >> a[i].first >> a[i].second;
  sort(a, a + m);
  a[m].first = d;
  curr.push(m);
  for (int i = m - 1; i >= 0; i--) {
    while (curr.size() > 1 && a[curr.top()].second >= a[i].second) curr.pop();
    nx[i] = curr.top();
    curr.push(i);
  }
  int fuel = n;
  long long cost = 0;
  for (int i = 0; i <= m; i++) {
    fuel -= a[i].first - (i ? a[i - 1].first : 0);
    if (fuel < 0) {
      cout << -1 << endl;
      return 0;
    }
    if (i == m) break;
    int x = min(n - fuel, max(0, a[nx[i]].first - a[i].first - fuel));
    fuel += x;
    cost += a[i].second * 1ll * x;
  }
  cout << cost << endl;
}
