#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 10;
struct node {
  int x, p;
} a[N];
int d, n, m, top, xx, nexttttt[N], last, q[N];
long long ans;
bool cmp(node a, node b) { return a.x < b.x; }
int main() {
  scanf("%d%d%d", &d, &n, &m);
  for (int i = 1; i <= m; i++) scanf("%d%d", &a[i].x, &a[i].p);
  sort(a + 1, a + m + 1, cmp);
  a[m + 1].x = d;
  a[m + 1].p = 0;
  for (int i = m + 1; i >= 0; i--) {
    if (a[i].x - a[i - 1].x > n) {
      puts("-1");
      return 0;
    }
    while (top && a[i].p < a[q[top]].p) top--;
    nexttttt[i] = q[top];
    q[++top] = i;
  }
  for (int i = 0; i <= m; i++) {
    xx = max(0, min(a[nexttttt[i]].x - a[i].x, n) - last);
    ans += 1ll * xx * a[i].p;
    last -= a[i + 1].x - a[i].x - xx;
  }
  printf("%lld\n", ans);
  return 0;
}
