#include <bits/stdc++.h>
using namespace std;
const int MAXN = 200005;
const int INFTY = 1 << 30;
int D, N, M;
pair<int, int> pos[MAXN];
vector<int> S;
int nxt[MAXN];
int main() {
  scanf("%d %d %d", &D, &N, &M);
  for (int i = 0; i < M; i++) scanf("%d %d", &pos[i].first, &pos[i].second);
  sort(pos, pos + M);
  pos[M] = make_pair(D, 0);
  for (int i = M - 1; i >= 0; i--) {
    while (!S.empty() && pos[S.back()].second > pos[i].second) S.pop_back();
    nxt[i] = S.empty() ? M : S.back();
    S.push_back(i);
  }
  long long ans = 0;
  int cur = N - pos[0].first;
  for (int i = 0; i < M; i++) {
    if (cur < 0) {
      printf("-1\n");
      return 0;
    }
    int need = min(N, pos[nxt[i]].first - pos[i].first);
    int add = max(0, need - cur);
    ans += (long long)pos[i].second * add;
    cur += add - (pos[i + 1].first - pos[i].first);
  }
  if (cur < 0)
    printf("-1\n");
  else
    printf("%I64d\n", ans);
  return 0;
}
