#include <bits/stdc++.h>
using namespace std;
vector<string> vec_splitter(string s) {
  s += ',';
  vector<string> res;
  while (!s.empty()) {
    res.push_back(s.substr(0, s.find(',')));
    s = s.substr(s.find(',') + 1);
  }
  return res;
}
void debug_out(vector<string> __attribute__((unused)) args,
               __attribute__((unused)) int idx,
               __attribute__((unused)) int LINE_NUM) {
  cerr << '\n';
}
template <typename Head, typename... Tail>
void debug_out(vector<string> args, int idx, int LINE_NUM, Head H, Tail... T) {
  if (idx > 0)
    cerr << ", ";
  else
    cerr << "Line(" << LINE_NUM << ") ";
  stringstream ss;
  ss << H;
  cerr << args[idx] << " = " << ss.str();
  debug_out(args, idx + 1, LINE_NUM, T...);
}
void run() {
  ios_base::sync_with_stdio(false);
  cin.tie(nullptr), cout.tie(nullptr);
}
const int N = 2e3 + 3;
int n, m, a;
vector<int> umb(N);
long long dp[N][N];
vector<bool> raining(N, false);
long long solve(int i, int j) {
  if (i == a) return 0;
  long long &ret = dp[i][j + 1];
  if (~ret) return ret;
  ret = INT_MAX;
  if (j != -1 || !raining[i]) ret = solve(i + 1, j) + umb[j];
  if (!raining[i]) ret = min(solve(i + 1, -1), ret);
  if (umb[i]) ret = min(ret, solve(i + 1, i) + umb[i]);
  return ret;
}
int main() {
  run();
  cin >> a >> n >> m;
  for (int i = 0; i < n; ++i) {
    int l, r;
    cin >> l >> r;
    for (; l < r; l++) raining[l] = true;
  }
  for (int i = 0; i < m; ++i) {
    int x, p;
    cin >> x >> p;
    umb[x] = (umb[x] == 0 ? p : min(umb[x], p));
  }
  memset(dp, -1, sizeof(dp));
  long long sol = solve(0, -1);
  cout << (sol >= INT_MAX ? -1 : sol);
  return 0;
}
