#include<bits/stdc++.h>
#define int long long
using namespace std;
const int N=501000;
int n,W,tA=1;
vector<int>G[N];
int val[N];
int build(void)
{
	int u=++tA,siz;
	cin>>val[u]>>siz;
	for(int i=1;i<=siz;i++)G[u].push_back(build());
	return u;
}
int res=0;
bool cmp(int x,int y){
	return val[x]<val[y];
}
void DFS(int u)
{
	int sum=0;
	for(int i=0,v;i<G[u].size();i++)
	{
		v=G[u][i];
		DFS(v);sum+=val[v];
	}
	sort(G[u].begin(),G[u].end());
	int siz=G[u].size();
	while(sum>W)++res,sum-=val[G[u][siz-1]],--siz;
	//if(u==2)cout<<sum<<' '<<res<<endl;
	val[u]+=sum;
	res+=val[u]/W;val[u]%=W;
	return;
}
signed main()
{
	cin>>W;
	G[1].push_back(build());
	DFS(1);
	if(val[1])++res;
	cout<<res;
	return 0;
}