#include<bits/stdc++.h>
using namespace std;
#define int long long
const int N=300+10, inf=1e18;
int n, m, f[N], val[N], d[N];
vector<pair<int, int>> g[N];
void pre_dfs(int u){
for (auto &e:g[u]){
d[e.first]=d[u]+e.second;
pre_dfs(e.first);
}
}
void dfs(int u){
if (g[u].empty()){
val[u]=d[u];
return;
}
vector<int> adj;
for (auto &e:g[u]){
int v=e.first;
dfs(v);
f[u]+=f[v];
adj.push_back(val[v]);
}
sort(adj.begin(), adj.end());
val[u]=adj[((int)adj.size()-1)/2];
for (auto &e:g[u]) val[u]=max(val[u], val[e.first]-e.second);
for (auto &e:g[u]) f[u]+=abs(val[u]-val[e.first]);
}
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> m;
if (n==1){
vector<int> v;
for (int i=2; i<=n+m; ++i){
int p, c; cin >> p >> c;
v.push_back(c);
}
sort(v.begin(), v.end());
int val=v[m/2], ans=0;
for (auto &i:v) ans+=abs(i-val);
cout << ans << '\n';
return 0;
}
for (int i=2; i<=m+n; ++i){
int p, c; cin >> p >> c;
g[p].emplace_back(i, c);
}
pre_dfs(1);
dfs(1);
cout << f[1] << '\n';
return 0;
}
