#include <bits/stdc++.h>
using namespace std;
long long int cons;
long long int check(long long int a) {
  if (a >= cons) a %= cons;
  return a;
}
long long int check2(long long int a) {
  a %= cons;
  if (a < 0) a += cons;
  return a;
}
long long int fast_check(long long int a) {
  if (a >= cons) a -= cons;
  return a;
}
long long int GCD(long long int a, long long int b) {
  while (a && b) {
    a = a % b;
    if (a != 0) b = b % a;
  }
  return a + b;
}
long long int exp(long long int a, long long int n) {
  long long int ans = 1;
  a = check(a);
  while (n) {
    if (n & 1) ans = check(ans * a);
    a = check(a * a);
    n = (n >> 1);
  }
  return ans;
}
void print_pair(pair<int, int> p) {
  cout << p.first << " " << p.second << endl;
}
const int N = 2001;
long long int dp[N], dp2[N];
int position[N];
int israin[N];
long long int umbrella[N];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cons = 1e9 + 7;
  int a, n, m;
  cin >> a >> n >> m;
  for (int i = 1; i <= n; i++) {
    int l, r;
    cin >> l >> r;
    for (int j = l; j <= r; j++) israin[j] = i;
  }
  for (int i = 1; i <= m; i++) {
    int first, p;
    cin >> first >> p;
    umbrella[i] = p;
    if (position[first] && umbrella[position[first]] > p)
      position[first] = i;
    else if (!position[first])
      position[first] = i;
  }
  for (int i = 1; i <= m; i++) dp[i] = 1e9;
  for (int i = 0; i < a; i++) {
    long long int mini = 1e9;
    for (int j = 0; j <= m; j++) {
      mini = min(mini, dp[j]);
      dp2[j] = 1e9;
    }
    int s = 0;
    if (israin[i] && israin[i + 1] && israin[i] == israin[i + 1])
      s++;
    else
      dp2[0] = mini;
    for (; s <= m; s++) {
      dp2[s] = min(dp2[s], dp[s] + umbrella[s]);
    }
    if (position[i]) {
      dp2[position[i]] = min(dp2[position[i]], mini + umbrella[position[i]]);
    }
    for (int j = 0; j <= m; j++) {
      dp[j] = dp2[j];
    }
  }
  long long int mini = 1e9;
  for (int i = 0; i <= m; i++) mini = min(mini, dp[i]);
  if (mini == 1e9)
    cout << -1;
  else
    cout << mini;
}
