#include<bits/stdc++.h>
#define pb push_back
#define ii pair<int,int>
#define all(x) (x).begin(),(x).end()
#define sz(x) ((int)(x).size())
#define INF 100000000000000000
#define modulo 1000000007
#define mod 998244353
#define int long long int
using namespace std;
int n, m;
vector<ii> adj[300005];
int g[300005][305];
void dfs(int x, int c, int d){
for(int i = 1; i <= 300; i++) g[x][i] = 0;
if(x > n)
for(int i = 1; i <= 300; i++){
if(i == d) g[x][i] = 0;
else g[x][i] = INF;
}
for(auto p : adj[x]){
dfs(p.first, p.second, d + p.second);
for(int a = 1; a <= 300; a++){
g[x][a] += g[p.first][a];
}
}
vector<int> f(305, INF);
for(int j = 1; j <= 300; j++){
for(int k = 1; k <= j + c && k <= 300; k++){
f[j] = min(f[j], g[x][k] + abs(k - j));
}
}
for(int i = 1; i <= 300; i++) g[x][i] = f[i];
}
int32_t main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> n >> m;
for(int i = 2; i <= n + m; i++){
int x, c;
cin >> x >> c;
adj[x].pb({i, c});
}
dfs(1, 0, 0);
int ans = INF;
for(int i = 1; i <= 300; i++) ans = min(ans, g[1][i]);
cout << ans;
}
