// only miss the sun when it starts to snow
#include<bits/stdc++.h>
#define F first
#define S second
#define PB push_back
#define sz(s) int((s).size())
#define bit(n,k) (((n)>>(k))&1)
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
const int maxn=1e5+10,mod=1e9+7;
const ll inf=1e18;
vector<pii> v[maxn];
vector<ll> vec;
pll s[maxn];
ll ans;
void dfs(int u){
for(pii p:v[u]){
dfs(p.F);
s[p.F].F+= p.S;
s[p.F].S+= p.S;
}
vec.clear();
ll num=0;
for(pii p:v[u]){
num+= s[p.F].S - s[p.F].F;
vec.PB( s[p.F].F ), vec.PB( s[p.F].S );
}
sort(vec.begin(), vec.end() );
if(sz(vec)){
int mid=sz(vec)/2;
s[u]={ vec[mid-1], vec[mid] };
ans-=num;
for(ll x:vec){
ans+= abs( s[u].F - x );
}
}
}
int main(){
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie();
int n,m; cin>>n>>m;
for(int i=1;i<n+m;i++){
int x,y; cin>>x>>y;
v[--x].PB({i,y});
}
dfs(0);
return cout<<(ans/2)<<endl,0;
}
// Deathly mistakes:
// * Read the problem carefully.
// * Check maxn.
// * Overflows.
// #pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
