#include <bits/stdc++.h>
using namespace std;
const long long maxn = 2010, INF = 1e13;
long long a, n, m, line[maxn], um[maxn], tot, dp[maxn];
map<long long, long long> mp, rmp;
vector<long long> dis;
int main() {
  scanf("%lld%lld%lld", &a, &n, &m);
  for (long long i = 0; i < n; ++i) {
    long long l, r;
    scanf("%lld%lld", &l, &r);
    for (long long j = l; j < r; ++j) line[j] = 1;
  }
  for (long long i = 0; i < m; ++i) {
    long long x, p;
    scanf("%lld%lld", &x, &p);
    if (um[x] == 0)
      um[x] = p;
    else
      um[x] = min(um[x], p);
  }
  for (long long i = 0; i < a; ++i)
    if (um[i]) dis.push_back(um[i]);
  sort(dis.begin(), dis.end());
  rmp[0] = 0;
  for (long long i = 0; i < dis.size(); ++i)
    if (i == 0 || dis[i] != dis[i - 1]) {
      mp[dis[i]] = ++tot;
      rmp[tot] = dis[i];
    }
  for (long long i = 0; i < a; ++i)
    if (um[i]) um[i] = mp[um[i]];
  for (long long i = 0; i <= tot; ++i) dp[i] = INF;
  dp[0] = 0;
  if (um[0]) dp[um[0]] = 0;
  for (long long i = 1; i <= a; ++i) {
    if (line[i - 1]) {
      dp[0] = INF;
      for (long long j = 1; j <= tot; ++j) dp[j] += rmp[j];
    } else {
      for (long long j = 0; j <= tot; ++j) dp[j] += rmp[j];
    }
    if (i == a) break;
    for (long long j = 0; j <= tot; ++j) {
      dp[0] = min(dp[0], dp[j]);
      if (um[i]) dp[um[i]] = min(dp[um[i]], dp[j]);
    }
  }
  long long ans = INF;
  for (long long i = 0; i <= tot; ++i) ans = min(ans, dp[i]);
  printf("%lld\n", ans >= INF ? -1 : ans);
  return 0;
}
