#include<cstdio>
#include<iostream>
#include<vector>
#include<algorithm>
#define RI register int
#define CI const int&
using namespace std;
int W,ans;
inline int DFS(void)
{
    int x,y; scanf("%d%d",&x,&y);
    vector <int> vec(y);
    for (RI i=0;i<y;++i)
    {
        int tmp=DFS(); ans+=tmp/W; vec[i]=tmp%W;
    }
    sort(vec.begin(),vec.end());
    int sum=0;
    for (RI i=0;i<y;++i)
    if (sum+vec[i]<=W) sum+=vec[i]; else ++ans;
    if (sum!=0) ++ans,x=max(0,x-(W-sum));
    return x;
}
int main()
{
    scanf("%d",&W);
    ans+=(DFS()+W-1)/W;
    return printf("%d",ans),0;
}