#include <bits/stdc++.h>
using namespace std;
const int MOD = (int)1e9;
long double sqr(long double a) { return a * a; }
long long gcd(long long a, long long b) {
  if (b == 0)
    return a;
  else
    return gcd(b, a % b);
}
long long dp[2001][2001];
pair<long long, long long> umb[2001];
bool rain[2001];
int main() {
  ios_base::sync_with_stdio(0);
  cout.tie(0);
  cin.tie(0);
  long long a, n, m, l, r;
  cin >> a >> n >> m;
  fill(rain, rain + 2002, false);
  for (auto i = 0; i < a + 1; ++i)
    for (auto j = 0; j < m + 1; ++j) dp[i][j] = INT32_MAX >> 2;
  for (auto i = 0; i < n; ++i) {
    cin >> l >> r;
    for (auto j = l + 1; j < r + 1; ++j) rain[j] = true;
  }
  for (auto i = 1; i < m + 1; ++i) {
    cin >> l >> r;
    umb[i] = {l + 1, r};
  }
  dp[0][0] = 0;
  for (auto i = 1; i < a + 1; ++i) {
    for (auto j = 1; j < m + 1; ++j) {
      if (umb[j].first == i)
        dp[i][j] = dp[i - 1][0] + umb[j].second;
      else
        dp[i][j] = dp[i - 1][j] + umb[j].second;
      if (rain[i])
        dp[i][0] = min(dp[i][0], dp[i][j]);
      else
        dp[i][0] = dp[i - 1][0];
    }
  }
  if (dp[a][0] > INT32_MAX >> 3)
    cout << -1;
  else
    cout << dp[a][0];
  return 0;
}
