P1280 尼克的任务

#include<iostream>
#include<algorithm>
using namespace std;
long int n,k,sum[10001],num=1,f[10001];
struct ren{
	long int ks,js;
}; 
ren z[10001];
int cmp(ren a,ren b)
{
	return a.ks>b.ks;
}
int main()
{
	long int i,j;
	cin>>n>>k;
	for(i=1;i<=k;i++)
	 {
	 	cin>>z[i].ks>>z[i].js;
	 	sum[z[i].ks]++;
	 }
	 sort(z+1,z+k+1,cmp);
	 for(i=n;i>=1;i--)
	 {
	 	if(sum[i]==0)
	 	 f[i]=f[i+1]+1;
	 	else
	 	 {
	 	 	for(j=1;j<=sum[i];j++,num++)
	 	    if(f[i+z[num].js]>f[i])
	 	      f[i]=f[i+z[num].js];
	 	      
		  }
	 }
	 cout<<f[1]<<endl;
	 return 0;
}