#include <bits/stdc++.h>
using namespace std;
const int N = 2111;
const long long INF = 1LL << 50;
bool good[N];
int at[N];
long long weight[N], cr[N], pr[N];
int main() {
  int a, n, m;
  scanf("%d%d%d", &a, &n, &m);
  for (int i = 0; i <= a; i++) {
    good[i] = true;
    at[i] = m;
  }
  while (n--) {
    int l, r;
    scanf("%d%d", &l, &r);
    for (int i = l; i < r; i++) good[i] = false;
  }
  for (int i = 0; i < m; i++) {
    int p;
    scanf("%d%lld", &p, &weight[i]);
    if (at[p] == m || weight[at[p]] > weight[i]) at[p] = i;
  }
  for (int i = 0; i <= m; i++) cr[i] = 0;
  for (int p = a; p--;) {
    for (int i = 0; i <= m; i++) pr[i] = cr[i], cr[i] = INF;
    for (int i = 0; i <= m; i++) {
      cr[i] = INF;
      for (int j : {m, i, at[p]}) {
        if (good[p] || j < m) cr[i] = min(cr[i], weight[j] + pr[j]);
      }
    }
  }
  printf("%lld\n", cr[m] < INF ? cr[m] : -1);
}
