#include <bits/stdc++.h>
using namespace std;
const int N = 2e3 + 5;
bool mark[N];
long long mp[N];
long long dp[N];
int main() {
  ios_base::sync_with_stdio(), cout.tie(NULL), cin.tie(NULL);
  ;
  long long a, n, m;
  cin >> a >> n >> m;
  memset(mark, false, sizeof(mark));
  for (int i = 0; i < n; i++) {
    int x, y;
    cin >> x >> y;
    for (int j = x; j < y; j++) {
      mark[j] = true;
    }
  }
  for (int i = 0; i <= a; i++) mp[i] = 1e6, dp[i] = 1e9;
  for (int i = 0; i < m; i++) {
    long long a, b;
    cin >> a >> b;
    mp[a] = min(b, mp[a]);
  }
  dp[0] = 0;
  for (int i = 1; i <= a; i++) {
    if (!mark[i - 1]) {
      dp[i] = dp[i - 1];
    } else {
      for (int j = i - 1; j >= 0; j--) {
        if (mp[j] < 1e6) dp[i] = min(dp[i], dp[j] + (i - j) * mp[j]);
      }
    }
  }
  if (dp[a] == 1e9) {
    cout << -1 << endl;
  } else
    cout << dp[a] << endl;
  return 0;
}
