#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
const int INF = 0x3f3f3f3f;
const long long LL_INF = 0x3f3f3f3f3f3f3f3f;
const long double PI = acos(-1);
const int MAXA = 2000 + 7;
long long dp[MAXA][MAXA];
bitset<MAXA> pada;
int waga[MAXA];
int parasol[MAXA];
int a, m, n;
long long dfs(int i, int u) {
  if (i == a) {
    return 0;
  }
  if (dp[i][u] != -1) {
    return dp[i][u];
  }
  dp[i][u] = LL_INF;
  if (pada[i]) {
    if (u != n) {
      dp[i][u] = min(dp[i][u], waga[u] + dfs(i + 1, u));
    }
  } else {
    if (u != n) {
      dp[i][u] = min(dp[i][u], waga[u] + dfs(i + 1, u));
    }
    dp[i][u] = min(dp[i][u], dfs(i + 1, n));
  }
  if (parasol[i] != -1) {
    dp[i][u] = min(dp[i][u], waga[parasol[i]] + dfs(i + 1, parasol[i]));
  }
  return dp[i][u];
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  cin >> a >> m >> n;
  for (int i = 0; i < m; i++) {
    int l, p;
    cin >> l >> p;
    for (int j = l; j < p; j++) {
      pada[j] = true;
    }
  }
  fill(parasol, parasol + MAXA, -1);
  for (int i = 0; i < n; i++) {
    int x, w;
    cin >> x >> w;
    waga[i] = w;
    if (parasol[x] == -1) {
      parasol[x] = i;
    } else if (waga[parasol[x]] > w) {
      parasol[x] = i;
    }
  }
  memset(dp, -1, sizeof(dp));
  long long res = dfs(0, n);
  if (res == LL_INF) res = -1;
  cout << res;
  return 0;
}
