#include <bits/stdc++.h>
using namespace std;
int rain[100005];
int um[100005];
int dp[100005];
int inf = 1e9;
int main() {
  int a, n, m;
  cin >> a >> n >> m;
  for (int i = 1; i <= n; i++) {
    int l, r;
    scanf("%d%d", &l, &r);
    for (int j = l; j <= r; j++) rain[j] = i;
  }
  for (int i = 0; i <= 100000; i++) {
    dp[i] = inf;
    um[i] = inf;
  }
  for (int i = 1; i <= m; i++) {
    int x, w;
    scanf("%d%d", &x, &w);
    um[x] = min(um[x], w);
  }
  dp[0] = 0;
  for (int i = 0; i < a; i++) {
    if (rain[i] == rain[i + 1] && rain[i]) {
      for (int j = 0; j <= i; j++) {
        if (um[j] != inf)
          dp[i + 1] = min(dp[i + 1], dp[j] + (i + 1 - j) * um[j]);
      }
    } else
      dp[i + 1] = dp[i];
  }
  if (dp[a] == inf)
    printf("-1\n");
  else
    printf("%d\n", dp[a]);
  return 0;
}
