#include <bits/stdc++.h>
using namespace std;
const int inf = 2000000000;
void io() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  cout.precision(15);
}
long long mulmod(long long a, long long b, long long m) {
  long long q = (long long)(((long double)a * (long double)b) / (long double)m);
  long long r = a * b - q * m;
  if (r > m) r %= m;
  if (r < 0) r += m;
  return r;
}
template <typename T>
T mod(T a, T b) {
  while (a < 0) a += b;
  return a % b;
}
template <typename T>
T power(T e, T n, T m) {
  T x = 1, p = e;
  while (n) {
    if (n & 1) x = mod(x * p, m);
    p = mod(p * p, m);
    n >>= 1;
  }
  return x;
}
template <typename T>
T power(T e, T n) {
  T x = 1, p = e;
  while (n) {
    if (n & 1) x = x * p;
    p = p * p;
    n >>= 1;
  }
  return x;
}
template <typename T>
T InverseEuler(T a, T m) {
  return (a == 1 ? 1 : power(a, m - 2, m));
}
template <typename T>
T gcd(T a, T b) {
  return __gcd(a, b);
}
template <typename T>
T lcm(T a, T b) {
  return (a * (b / gcd(a, b)));
}
int exEuler(int a, int b, int &x, int &y) {
  if (a == 0) {
    x = 0;
    y = 1;
    return b;
  }
  if (b == 0) {
    x = 1;
    y = 0;
    return a;
  }
  int x1, y1;
  int g = exEuler(b % a, a, x1, y1);
  x = y1 - (b / a) * x1;
  y = x1;
  return g;
}
pair<int, int> x[200020];
int next_halt[200020];
int main(int argc, char *argv[]) {
  io();
  int d, n, m;
  cin >> d >> n >> m;
  for (__typeof(m) i = 1; i <= m; ++i) {
    cin >> x[i].first >> x[i].second;
  }
  x[m + 1].first = d;
  x[m + 1].second = 0;
  sort(x + 1, x + m + 2);
  if (n < x[1].first) {
    cout << -1 << '\n';
    return 0;
  }
  if (n == d) {
    cout << 0 << '\n';
    return 0;
  }
  bool f = 1;
  stack<int> st;
  st.push(m + 1);
  for (__typeof(1) i = m; i >= 1; --i) {
    while (x[st.top()].second > x[i].second) st.pop();
    next_halt[i] = st.top();
    st.push(i);
  }
  int cur = n;
  int pos = 0, prev_pos = 0;
  x[0].first = 0;
  x[0].second = inf;
  long long ans = 0;
  for (__typeof(m + 1) i = 1; i <= m + 1; ++i) {
    int toTravel = x[i].first - x[pos].first;
    if (cur < toTravel) {
      f = 0;
      break;
    }
    cur = cur - toTravel;
    int nextDis = x[next_halt[i]].first - x[i].first;
    nextDis = min(n, nextDis);
    if (cur < nextDis) {
      ans = ans + 1ll * (nextDis - cur) * x[i].second;
      cur = nextDis;
    }
    pos = i;
  }
  if (!f) {
    cout << -1 << '\n';
    return 0;
  }
  cout << ans << '\n';
  return 0;
}
