#include <bits/stdc++.h>
using namespace std;
int a, n, m;
int l[2005], r[2005];
int v[2005];
int dp[2005];
int p[2005], w[2005];
int main() {
  cin >> a >> n >> m;
  for (int i = 1; i <= n; i++) {
    cin >> l[i] >> r[i];
  }
  for (int i = 1; i <= m; i++) {
    cin >> p[i] >> w[i];
  }
  memset(dp, 0x3f3f3f3f, sizeof(dp));
  dp[0] = 0;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= m; j++) {
      if (p[j] <= l[i] && p[j] >= r[i - 1]) {
        for (int k = p[j] + 1; k <= r[n]; k++) {
          dp[k] = min(dp[k], dp[r[i - 1]] + w[j] * (k - p[j]));
        }
      }
    }
    for (int j = 1; j <= m; j++) {
      if (p[j] >= l[i] && p[j] <= r[i]) {
        for (int k = p[j] + 1; k <= r[n]; k++) {
          dp[k] = min(dp[k], dp[p[j]] + w[j] * (k - p[j]));
        }
      }
    }
  }
  if (dp[r[n]] != 0x3f3f3f3f)
    cout << dp[r[n]];
  else
    cout << -1;
}
