#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
#define X first
#define Y second
#define SZ(x) int(x.size())
#define pb push_back
#define mins(a,b) (a=min(a,b))
vector<pll> merge(vector<pll> &f, vector<pll> &g) {
assert(SZ(f)>=3 && SZ(g)>=3);
vector<pll> h;
ll fs=(f[0].Y-f[1].Y)/(f[0].X-f[1].X);
ll gs=(g[0].Y-g[1].Y)/(g[0].X-g[1].X);
int i=0, j=0;
while(i<SZ(f) && j<SZ(g)) {
if(f[i].X<g[j].X) {
h.pb(pll(f[i].X, f[i].Y+g[j].Y+(f[i].X-g[j].X)*gs));
i++;
if(i==SZ(f)) fs = (f[i-1].Y-f[i-2].Y)/(f[i-1].X-f[i-2].X);
else fs = (f[i-1].Y-f[i].Y)/(f[i-1].X-f[i].X);
}
else if(f[i].X>g[j].X) {
h.pb(pll(g[j].X, g[j].Y+f[i].Y+(g[j].X-f[i].X)*fs));
j++;
if(j==SZ(g)) gs = (g[j-1].Y-g[j-2].Y)/(g[j-1].X-g[j-2].X);
else gs = (g[j-1].Y-g[j].Y)/(g[j-1].X-g[j].X);
}
else {
h.pb(pll(f[i].X, f[i].Y+g[j].Y));
i++;
j++;
if(i==SZ(f)) fs = (f[i-1].Y-f[i-2].Y)/(f[i-1].X-f[i-2].X);
else fs = (f[i-1].Y-f[i].Y)/(f[i-1].X-f[i].X);
if(j==SZ(g)) gs = (g[j-1].Y-g[j-2].Y)/(g[j-1].X-g[j-2].X);
else gs = (g[j-1].Y-g[j].Y)/(g[j-1].X-g[j].X);
}
}
while(i<SZ(f)) {
h.pb(pll(f[i].X, f[i].Y+g[j-1].Y+(f[i].X-g[j-1].X)*gs));
i++;
}
while(j<SZ(g)) {
h.pb(pll(g[j].X, g[j].Y+f[i-1].Y+(g[j].X-f[i-1].X)*fs));
j++;
}
return h;
}
vector<pll> merge(vector<vector<pll>> F) {
vector<vector<pll>> G;
while(SZ(F)>1) {
for(int i=1; i<SZ(F); i+=2) G.pb(merge(F[i-1], F[i]));
if(SZ(F)&1) G.pb(F.back());
F = G;
G.clear();
}
return F[0];
}
void add_par(vector<pll> &f, int w) {
while(f.back().Y>f[SZ(f)-2].Y) f.pop_back();
vector<pll> stk;
while(f.back().Y==f[SZ(f)-2].Y) {
stk.pb(f.back());
f.pop_back();
}
stk.pb(f.back());
for(auto &i : f) i.Y+=w;
while(!stk.empty()) {
f.pb(pll(stk.back().X+w, stk.back().Y));
stk.pop_back();
}
f.pb(pll(f.back().X+1, f.back().Y+1));
}
const int MXN = 5003;
int n, m;
vector<pii> g[MXN];
vector<pll> f[MXN];
void dfs(int v, int pw=-1) {
if(v>n) {
f[v] = {{pw-1, 1}, {pw, 0}, {pw+1, 1}};
return;
}
vector<vector<pll>> F;
for(auto [u, w] : g[v]) {
dfs(u, w);
F.pb(f[u]);
}
f[v] = merge(F);
if(pw!=-1) add_par(f[v], pw);
}
int32_t main() {
cin.tie(0); cout.tie(0); ios_base::sync_with_stdio(0);
cin >> n >> m;
for(int i=2,p,c; i<=n+m; i++) {
cin >> p >> c;
g[p].pb(pii(i, c));
}
dfs(1);
ll ans = 2e18;
for(auto [x, y] : f[1])
mins(ans, y);
cout << ans << '\n';
return 0;
}
