#include <bits/stdc++.h>
using namespace std;
const int INF = 1000 * 1000 * 1000;
int main() {
  int a, n, m;
  cin >> a >> n >> m;
  vector<int> rain(a + 1);
  vector<pair<int, int>> umb(a + 1, make_pair(INF, -1));
  vector<int> costs(m);
  for (int i = 0; i < n; ++i) {
    int l, r;
    cin >> l >> r;
    for (int j = l; j < r; ++j) rain[j] = 1;
  }
  for (int i = 0; i < m; ++i) {
    int x, p;
    cin >> x >> p;
    costs[i] = p;
    umb[x] = min(umb[x], make_pair(p, i));
  }
  vector<vector<int>> dp(a + 1, vector<int>(m + 1, INF));
  dp[0][m] = 0;
  for (int i = 0; i < a; ++i) {
    for (int j = 0; j <= m; ++j) {
      if (dp[i][j] == INF) continue;
      if (rain[i] == 0) dp[i + 1][m] = min(dp[i + 1][m], dp[i][j]);
      if (j < m) dp[i + 1][j] = min(dp[i + 1][j], dp[i][j] + costs[j]);
      if (umb[i].first != INF)
        dp[i + 1][umb[i].second] =
            min(dp[i + 1][umb[i].second], dp[i][j] + umb[i].first);
    }
  }
  int ans = INF;
  for (int i = 0; i <= m; ++i) ans = min(ans, dp[a][i]);
  if (ans == INF) ans = -1;
  cout << ans << endl;
  return 0;
}
