#include <bits/stdc++.h>
using namespace std;
inline int read() {
  int x = 0, f = 1;
  char c = getchar();
  while (c < '0' || c > '9') {
    if (c == '-') f = -1;
    c = getchar();
  }
  while (c >= '0' && c <= '9')
    x = (x << 3) + (x << 1) + (c ^ '0'), c = getchar();
  return x * f;
}
const int N = 2e5 + 5;
int d, n, m;
struct node {
  int x, p;
} a[N];
inline int operator<(const node x, const node y) {
  if (x.p == y.p) return x.x > y.x;
  return x.p > y.p;
}
inline int cmp(node x, node y) { return x.x < y.x; }
priority_queue<node> q;
long long ans;
int main() {
  d = read(), n = read(), m = read();
  for (int i = 1; i <= m; i++) a[i] = {read(), read()};
  sort(a + 1, a + m + 1, cmp);
  int t = n, j = 1;
  while (t < d) {
    while (j <= m && a[j].x <= t) q.push(a[j++]);
    while (!q.empty() && q.top().x + n <= t) q.pop();
    if (q.empty()) return puts("-1"), 0;
    int x = q.top().x, p = q.top().p;
    int nxt = min(d, x + n);
    if (j <= m && nxt >= a[j].x) nxt = a[j].x;
    ans += 1ll * p * (nxt - t);
    t = nxt;
  }
  printf("%lld\n", ans);
  return 0;
}
