#include<bits/stdc++.h>

 using namespace std;

 const int N = 1e5;

 const int INF = 0x3f3f3f3f;

 int rain[N];

 int w[N];

 int dp[N];

 int main(){

     int a, n, m, l, r;

     scanf("%d%d%d", &a, &n, &m);

     for(int i = ; i <= n; i++){

         scanf("%d%d", &l, &r);

         for(int j = l; j <= r; j++)

             rain[j] = i;

     }

     memset(dp, INF, sizeof(dp));

     memset(w, INF, sizeof(w));

     for(int i = ; i <= m; i++){

         scanf("%d%d", &l, &r);

         w[l] = min(w[l], r);

     }

     dp[] = ;

     for(int i = ; i < a; i++){

         if(rain[i] == rain[i+]  && rain[i]){

             for(int j = ; j <= i; j++)

                 if(w[j]!=INF) dp[i+] = min(dp[i+], dp[j]+(i+-j)*w[j]);

         }

         else dp[i+] = dp[i];

     }

     printf("%d\n",dp[a]>=INF ? - : dp[a]);

 }