#include<bits/stdc++.h>
using namespace std;
#define int long long
const int N=5000+10, inf=1e18;
int n, m, f[N][N], d[N];
vector<int> vals;
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()){
f[u][lower_bound(vals.begin(), vals.end(), d[u])-vals.begin()]=0;
return;
}
for (int i=0; i<N; ++i) f[u][i]=0;
for (auto &e:g[u]){
int v=e.first, w=e.second;
dfs(v);
deque<int> dq;
for (int i=1, j=1; i<(int)vals.size(); ++i){
while (dq.size() && dq.front()<i) dq.pop_front();
while (j<(int)vals.size() && vals[j]-vals[i]<=w){
while (dq.size() && f[v][dq.back()]+vals[dq.back()]>=f[v][j]+vals[j]) dq.pop_back();
dq.push_back(j);
++j;
}
f[v][i]=min(f[v][i], f[v][dq.front()]+vals[dq.front()]-vals[i]);
}
for (int i=2; i<(int)vals.size(); ++i) f[v][i]=min(f[v][i], f[v][i-1]+vals[i]-vals[i-1]);
for (int i=1; i<(int)vals.size(); ++i) f[u][i]=min(inf, f[u][i]+f[v][i]);
}
}
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);
}
for (int i=0; i<N; ++i) for (int j=0; j<N; ++j) f[i][j]=inf;
pre_dfs(1);
vals.push_back(-1);
for (int i=n+1; i<=n+m; ++i) vals.push_back(d[i]);
sort(vals.begin(), vals.end()); vals.resize(unique(vals.begin(), vals.end())-vals.begin());
dfs(1);
cout << *min_element(f[1]+1, f[1]+(int)vals.size()) << '\n';
return 0;
}
