#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long int a, n, m, x, y;
  cin >> a >> n >> m;
  vector<long long int> v(a + 1, INT_MAX);
  vector<long long int> rain(a + 1);
  vector<long long int> umbrella(a + 1, INT_MAX);
  for (int i = 0; i < n; i++) {
    cin >> x >> y;
    rain[x]++;
    rain[y]--;
  }
  for (int i = 1; i <= a; i++) rain[i] += rain[i - 1];
  for (int i = 0; i < m; i++) {
    cin >> x >> y;
    umbrella[x] = min(umbrella[x], y);
  }
  v[a] = 0;
  for (int i = a - 1; i >= 0; i--) {
    if (!rain[i]) v[i] = min(v[i], v[i + 1]);
    if (umbrella[i] != INT_MAX) {
      for (int j = i + 1; j <= a; j++) {
        v[i] = min(v[i], (j - i) * umbrella[i] + v[j]);
      }
    }
  }
  if (v[0] == INT_MAX)
    cout << "-1\n";
  else
    cout << v[0] << '\n';
}
int main() {
  int t = 1;
  for (int i = 1; i <= t; i++) solve();
  return 0;
}
