#include<bits/stdc++.h>
#define ll long long
#define ff first
#define ss second
#define pb push_back
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
using namespace std;
vector<pair<ll,ll>>adj[200005];
vector<ll>v;
vector<ll>sz(200005);
ll calc(ll x,ll p){
sz[x] = 1;
for(auto [y,w] : adj[x]){
if(y!=p){
sz[x]+=calc(y,x);
}
}
return sz[x];
}
int main(){
ll n,m;
cin >> n >> m;
for(ll i = 2; i <= n; i++){
ll a,c;
cin >> a >> c;
adj[a].pb({i,c});
adj[i].pb({a,c});
}
ll ans = 0;
vector<pair<ll,ll>>bomba;
for(ll i = n+1; i <= n+m; i++){
ll a,c;
cin >> a >> c;
adj[a].pb({i,c});
bomba.pb({c,i});
}
sort(all(bomba));
for(ll i = 0; i < bomba.size(); i++){
ans+= abs(bomba[m/2].ff-bomba[i].ff);
}
cout << ans;
}
