法1

#include<iostream>
#include<algorithm>
#include<cstring>
#include<unordered_map>
#include<vector>

using namespace std;

typedef long long LL;

const int N = 5*100010;

LL n,a,b;
unordered_map<LL,LL> mp1,mp2;
LL sum = 0;

int main()
{
    cin>>n>>a>>b;
    LL l,r;
    while(a--)
    {
        cin>>l>>r;
        mp1[l]=r;
    }
    while(b--)
    {
        cin>>l>>r;
        mp2[l]=r;
    }
    for(auto i=mp1.begin(); i!=mp1.end(); i++)
    {
        //cout<<i->first<<' ';
        if(mp2.count((int)i->first)!=0)
        {
            sum+=mp1[i->first]*mp2[i->first];
            //cout<<sum<<endl;
        }
    }
    cout<<sum;
    return 0;
}

法2

#include<iostream>
#include<algorithm>
#include<cstring>
#include<unordered_map>
#include<vector>

using namespace std;

typedef long long LL;

LL n,a,b;
unordered_map<LL,LL> mp;
LL sum = 0;

int main()
{
    cin>>n>>a>>b;
    LL l,r;
    while(a--)
    {
        cin>>l>>r;
        mp[l]=r;
    }
    while(b--)
    {
        cin>>l>>r;
        if(mp[l])
        {
            sum+=mp[l]*r;
        }
    } 
    cout<<sum;
    return 0;
}