#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:268435456")
using namespace std;
template <typename T>
inline T abs(T a) {
  return ((a < 0) ? -a : a);
}
template <typename T>
inline T sqr(T a) {
  return a * a;
}
template <class T>
T gcd(T a, T b) {
  return a ? gcd(b % a, a) : b;
}
template <class T>
T lcm(T a, T b) {
  return a / gcd(a, b) * b;
}
template <class T>
T sign(T a) {
  return a > 0 ? 1 : (a < 0 ? -1 : 0);
}
const int dx[] = {-1, 0, 1, 0};
const int dy[] = {0, 1, 0, -1};
const int dxK[] = {-1, -1, 0, 1, 1, 1, 0, -1};
const int dyK[] = {0, 1, 1, 1, 0, -1, -1, -1};
const int dxKn[] = {-2, -1, 1, 2, 2, 1, -1, -2};
const int dyKn[] = {1, 2, 2, 1, -1, -2, -2, -1};
const int N = int(1e6) + 9;
const int M = int(2e2) + 9;
const int LOGN = 22;
const int SQN = 350;
const int MOD = int(1e9) + 7;
const int INF = 1e9 + 100;
const long long INF64 = 2e18;
const long double PI = 3.1415926535897932384626433832795;
const long double EPS = 1e-9;
long long d, v, n;
pair<long long, long long> p[N];
void solve() {
  scanf("%I64d %I64d %I64d", &d, &v, &n);
  for (int i = 0; i < (int)(n); ++i)
    scanf("%I64d %I64d", &p[i].first, &p[i].second);
  p[n++] = make_pair(0, 0);
  p[n++] = make_pair(d, 0);
  sort(p, p + n);
  p[n] = make_pair(d, 0);
  multiset<pair<long long, long long> > s;
  long long res = 0;
  long long remd = v;
  for (int i = 0; i < (int)(n); ++i) {
    long long dist = p[i + 1].first - p[i].first;
    if (dist > v) {
      cout << -1;
      return;
    }
    long long cost = p[i].second;
    while (!s.empty() && s.rbegin()->first > cost) {
      pair<long long, long long> cur = *s.rbegin();
      s.erase(--s.end());
      res -= cur.first * 1LL * cur.second;
      remd += cur.second;
    }
    if (remd) {
      res += cost * remd;
      s.insert(make_pair(cost, remd));
      remd = 0;
    }
    while (dist > 0) {
      pair<long long, long long> cur = *s.begin();
      s.erase(s.begin());
      long long curdist = min(dist, cur.second);
      dist -= curdist;
      cur.second -= curdist;
      remd += curdist;
      if (cur.second > 0) s.insert(cur);
    }
  }
  cout << res;
}
int main() {
  srand(time(NULL));
  srand(time(NULL));
  cout << setprecision(10) << fixed;
  cerr << setprecision(10) << fixed;
  solve();
  return 0;
}
