#include <bits/stdc++.h>
using namespace std;
const long long INF = 2e9 + 10, MAX = 2100, MOD = 1e9 + 7, MAXL = 25;
void OUT(long double o, int x) {
  cout << fixed << setprecision(x) << o;
  return;
}
long long dp[MAX][MAX], ra[MAX], a[MAX], re[200000], w[MAX];
vector<int> v[MAX];
pair<long long, long long> c[MAX];
set<int> st;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int n, m, t;
  cin >> n >> m >> t;
  for (int i = 1; i <= m; i++) {
    int l, r;
    cin >> l >> r;
    for (int j = l + 1; j <= r; j++) ra[j] = 1;
  }
  for (int i = 1; i <= t; i++) {
    cin >> c[i].first >> c[i].second;
    st.insert(c[i].second);
  }
  int cnt = 0;
  for (int i = 1; i <= 2000; i++) w[i] = INF;
  for (int u : st) {
    re[u] = ++cnt;
    w[cnt] = u;
  }
  for (int i = 1; i <= t; i++) v[c[i].first].push_back(re[c[i].second]);
  for (int i = 1; i <= 2000; i++) dp[0][i] = INF;
  for (int u : v[0]) dp[0][u] = 0;
  for (int i = 1; i <= n; i++) {
    long long mi = INF;
    for (int j = 1; j <= 2000; j++) mi = min(mi, dp[i - 1][j] + w[j]);
    if (!ra[i]) mi = min(mi, dp[i - 1][0]);
    dp[i][0] = mi;
    for (int j = 1; j <= 2000; j++) dp[i][j] = dp[i - 1][j] + w[j];
    for (int u : v[i]) dp[i][u] = min(dp[i][u], mi);
  }
  long long ans = INF;
  for (int i = 0; i <= 2000; i++) ans = min(ans, dp[n][i]);
  if (ans < INF)
    cout << ans;
  else
    cout << -1;
  return 0;
}
