zoj 2734 Exchange Cards

#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;

#define MAXN 10000

struct Card
{
int value;
int num;
};

Card cards[MAXN];
int n,m;
int sum;

bool cmp(const Card&a ,const Card&b){
return a.value>b.value;
}

void dfs(int pos,int target)
{
if (target<0) return;
if (target==0){
sum++;
return;
}
if (pos>=m) return;
int dec = target-cards[pos].value*cards[pos].num;
for (int i=cards[pos].num;i>=0;i--){
dfs(pos+1,dec);
dec += cards[pos].value;
}
}

int main()
{
int ct =0;
while(scanf("%d %d",&n,&m)!=EOF){
if (ct==0)
ct++;
else
printf("\n");
for (int i=0;i<m;i++){
scanf("%d %d",&cards[i].value,&cards[i].num);
}
sort(cards,cards+m,cmp);
sum = 0;
dfs(0,n);
printf("%d\n",sum);
}
return 0;
}