#include <bits/stdc++.h>
using namespace std;
const int N = 2005;
int a, n, m, dp[N][N], rain[N], c[N];
pair<int, int> umb[N];
class SegmentTree {
 public:
  pair<int, int> node[4 * N];
  void init(int i, int l, int r) {
    if (l > r) return;
    if (l == r) {
      node[i] = umb[l];
      return;
    }
    int m = (l + r) >> 1;
    init(i << 1, l, m);
    init(i << 1 | 1, m + 1, r);
    node[i] = min(node[i << 1], node[i << 1 | 1]);
  }
  pair<int, int> get(int i, int l, int r, int a, int b) {
    if (l > r || a > r || b < l) return make_pair(0x3f3f3f3f, 0);
    if (a <= l && r <= b) return node[i];
    int m = (l + r) >> 1;
    return min(get(i << 1, l, m, a, b), get(i << 1 | 1, m + 1, r, a, b));
  }
} it;
int main() {
  scanf("%d %d %d", &a, &n, &m);
  memset(rain, 0, sizeof(rain));
  for (int i = 1; i <= n; i++) {
    int l, r;
    scanf("%d %d", &l, &r);
    for (int j = l; j < r; j++) rain[j]++;
  }
  memset(umb, 0x3f, sizeof(umb));
  for (int i = 1; i <= m; i++) {
    int pos, val;
    scanf("%d %d", &pos, &val);
    umb[pos] = min(umb[pos], make_pair(val, i));
    c[i] = val;
  }
  it.init(1, 0, a);
  memset(dp, 0x3f, sizeof(dp));
  dp[0][0] = 0;
  for (int i = 0; i < a; i++) {
    for (int j = 0; j <= m; j++) {
      if (dp[i][j] != 0x3f3f3f3f) {
        if (j) dp[i + 1][j] = min(dp[i + 1][j], dp[i][j] + c[j]);
        if (!rain[i]) dp[i + 1][0] = min(dp[i + 1][0], dp[i][j]);
        pair<int, int> pos = it.get(1, 0, a, i, i);
        if (pos.first != 0x3f3f3f3f) {
          dp[i + 1][pos.second] =
              min(dp[i + 1][pos.second], dp[i][j] + pos.first);
        }
      }
    }
  }
  int ans = 0x3f3f3f3f;
  for (int i = 0; i <= m; i++) ans = min(ans, dp[a][i]);
  if (ans == 0x3f3f3f3f)
    printf("-1");
  else
    printf("%d", ans);
  return 0;
}
