#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define vi vector<int>
#define pii pair<int,int>
#define pll pair<ll,ll>
#define vll vector<ll>
#define FOR(i,n) for(int i=0;i<n;i++)
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
#define pb push_back
#define f first
#define s second

const int MOD = (int)1e9+7;
ll w;
ll ans = 0;
ll create(){
    ll height,branches; 
    cin>>height>>branches;
    ll remain = 0;
    multiset<ll> br;
    FOR(i,branches){
        ll x = create();
        if(x)
        br.insert(x);
        remain+=x;
    }
    while(sz(br) && remain>w){
        ll rr = w;
        while(true){
           auto it = br.upper_bound(rr);
           if(it==br.begin()) break;
           it--;
           br.erase(it);
           rr-=w;
        }
        remain-=w-rr;
        ans++;
    }
    remain+=height;
    ll k = remain/w;
    ans+=k;
    remain-=k*w;
    return remain;
}

int main()
{
    cin>>w;
    ll x = create();
    if(x) ans++;
    cout<<ans<<endl;
    return 0;
}