#include <bits/stdc++.h>
using namespace std;
const int MX = 2005;
int dp[MX];
bool rain[MX];
int un[MX];
int main() {
  int a, m, n;
  cin >> a >> n >> m;
  while (n--) {
    int l, r;
    scanf("%d%d", &l, &r);
    for (int i = l; i < r; i++) rain[i] = 1;
  }
  for (int i = 1; i <= m; i++) {
    int sc1, sc2;
    scanf("%d%d", &sc1, &sc2);
    if (un[sc1])
      un[sc1] = min(sc2, un[sc1]);
    else
      un[sc1] = sc2;
  }
  for (int i = 0; i <= a; i++) {
    dp[i] = INT_MAX / 2;
    if (!rain[i]) {
      dp[i] = ((i == 0) ? 0 : dp[i - 1]);
    } else {
      for (int j = 0; j <= i; j++) {
        if (un[j]) {
          dp[i] = min(dp[i], (j == 0 ? 0 : dp[j - 1]) + (i - j + 1) * un[j]);
        }
      }
      if (dp[i] == INT_MAX / 2) {
        puts("-1");
        return 0;
      }
    }
  }
  cout << dp[a];
  return 0;
}
