#include <bits/stdc++.h>
using namespace std;
const int MX = 2005;
map<int, int> mp[2];
bool rain[MX];
int un[MX];
int main() {
  int a, m, n;
  cin >> a >> n >> m;
  while (n--) {
    int l, r;
    scanf("%d%d", &l, &r);
    for (int i = l; i < r; i++) rain[i] = 1;
  }
  for (int i = 1; i <= m; i++) {
    int sc1, sc2;
    scanf("%d%d", &sc1, &sc2);
    if (un[sc1])
      un[sc1] = min(sc2, un[sc1]);
    else
      un[sc1] = sc2;
  }
  int now = 0;
  mp[now][0] = 0;
  for (int i = 0; i <= a; i++) {
    if (mp[now].size() == 0) {
      puts("-1");
      return 0;
    }
    mp[now ^ 1].clear();
    int mi = INT_MAX;
    for (auto it : mp[now]) {
      mi = min(mi, it.second);
      if (it.first == 0 && rain[i]) {
        continue;
      }
      mp[now ^ 1][it.first] = it.second + it.first;
    }
    if (rain[i] == 0) mp[now ^ 1][0] = mi;
    if (un[i]) mp[now ^ 1][un[i]] = mi + un[i];
    now = now ^ 1;
  }
  int ans = INT_MAX;
  for (auto it : mp[now]) ans = min(ans, it.second);
  cout << ans;
  return 0;
}
