#include <bits/stdc++.h>
using namespace std;
#define POPCOUNT(n) (__builtin_popcountll((n)))
#define CLZ(n) (__builtin_clzll((n)))
#define CTZ(n) (__builtin_ctzll((n)))
#define LOG(n) (63 - __builtin_clzll((n)))
#define BIT(n, i) (((n) >> (i)) & 1ll)
#define MASK(i) (1ll << (i))
#define FLIP(n, i) ((n) ^ (1ll << (i)))
#define ON(n, i) ((n) | MASK(i))
#define OFF(n, i) ((n) & ~MASK(i))
#define Int __int128
#define fi first
#define se second
typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<long long, long long> pll;
typedef pair<long long, int> pli;
typedef pair<int, long long> pil;
typedef vector<pair<int, int>> vii;
typedef vector<pair<long long, long long>> vll;
typedef vector<pair<long long, int>> vli;
typedef vector<pair<int, long long>> vil;
template <class T1, class T2> bool maximize(T1 &x, T2 y) {
if (x < y) {
x = y;
return true;
}
return false;
}
template <class T1, class T2> bool minimize(T1 &x, T2 y) {
if (x > y) {
x = y;
return true;
}
return false;
}
template <class T> void remove_duplicate(vector<T> &ve) {
sort (ve.begin(), ve.end());
ve.resize(unique(ve.begin(), ve.end()) - ve.begin());
}
mt19937_64 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
template <class T1, class T2> T1 random(T1 l, T2 r) {
return uniform_int_distribution<T1>(l, r)(rng);
}
template <class T> T random(T r) {
return rng() % r;
}
const int N = 5000 + 5;
const int MOD = 1e9 + 7;
const int inf = 1e9;
const long long INF = 1e18;
int n, m;
int par[N], c[N];
ll dist[N], dp[N][N];
vii adj[N];
vector<int> leaf[N];
void prepare(int u) {
if (adj[u].empty()) leaf[u].emplace_back(u);
for (auto [v, w] : adj[u]) {
dist[v] = dist[u] + w;
prepare(v);
for (auto &x : leaf[v]) leaf[u].emplace_back(x);
}
}
void dfs(int u) {
if (adj[u].empty()) {
dp[u][0] = 0;
for (int x = 1; x <= 300; ++x) dp[u][x] = INF;
return;
}
for (auto [v, w] : adj[u]) {
dfs(v);
}
for (int x = 0; x <= 300; ++x) {
for (auto [v, w] : adj[u]) {
ll bestY = INF;
for (int y = 0; y <= 300; ++y) {
// w - dist[y] + dist[x]
if (w - y + x >= 0)
minimize(bestY, dp[v][y] + abs(x - y));
}
dp[u][x] += bestY;
}
// cerr << u << ' ' << x << ' ' << dp[u][x] << '\n';
}
}
signed main() {
ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
cin >> n >> m;
for (int i = 2; i <= n + m; ++i) {
cin >> par[i] >> c[i];
adj[par[i]].emplace_back(i, c[i]);
}
// cerr << '\n';
// for (int i = 2; i <= n + m; ++i)
// cerr << par[i] << ' ' << i << ' ' << c[i] << '\n';
// cerr << '\n';
prepare(1);
dfs(1);
ll ans = INF;
for (auto &x : leaf[1]) minimize(ans, dp[1][x]);
cout << ans << '\n';
return 0;
}
