#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
#define pb push_back
#define all(x) x.begin(), x.end()
#define SZ(x) int(x.size())
const int MXN = 3e5+5;
int n;
vector<pii> g[MXN];
ll dp[MXN], L[MXN], R[MXN];
void dfs(int v) {
vector<ll> vec;
for(auto [u, w] : g[v]) {
dfs(u);
dp[v] += dp[u];
vec.pb(L[u]+w);
vec.pb(R[u]+w);
}
sort(all(vec));
if(SZ(vec)&1) L[v] = R[v] = vec[SZ(vec)>>1];
else if(SZ(vec)) L[v] = vec[SZ(vec)/2-1], R[v] = vec[SZ(vec)>>1];
for(auto [u, w] : g[v])
dp[v] += (abs(L[u]+w-L[v]) + abs(R[u]+w-L[v]) - (R[u]-L[u])) / 2;
}
int32_t main() {
cin.tie(0); cout.tie(0); ios_base::sync_with_stdio(0);
int m;
cin >> n >> m;
n += m;
for(int i=2, p,c; i<=n; i++) {
cin >> p >> c;
g[p].pb(pii(i, c));
}
dfs(1);
cout << dp[1] << '\n';
}
