FatMouse' Trade

http://acm.hdu.edu.cn/showproblem.php?pid=1009

#include <map>
#include <cstdio>
#include <stack>
#include <cmath>
#include <queue>
#include <string>
#include <queue>
#include <cstring>
#include <iostream>
#include <algorithm>

#define lid id<<1
#define rid id<<1|1
#define closein cin.tie(0)
#define scac(a) scanf("%c",&a)
#define scad(a) scanf("%d",&a)
#define print(a) printf("%d
",a)
#define scald(a) scanf("%lld",&a)
#define debug printf("hello world")
#define form(i,n,m) for(int i=n;i<m;i++)
#define mfor(i,n,m) for(int i=n;i>m;i--)
#define nfor(i,n,m) for(int i=n;i>=m;i--)
#define forn(i,n,m) for(int i=n;i<=m;i++)
#define scadd(a,b) scanf("%d%d",&a,&b)
#define memset0(a) memset(a,0,sizeof(a))
#define scaldd(a,b) scanf("%lld%lld",&a,&b)
#define scaddd(a,b,c) scanf("%d%d%d",&a,&b,&c)
#define scadddd(a,b,c,d) scanf("%d%d%d%d",&a,&b,&c,&d)

#define INF 0x3f3f3f3f
#define maxn 50000
#define mod 1000000007
typedef long long ll;
using namespace std;
//---------AC(^-^)AC---------\

struct node
{
    int a,b;
    double x;
    bool operator < (const node& a)const
    {
        return x>a.x;
    }
}s[1005];
int main()
{
    int n,m;
    while(scadd(n,m))
    {
        if(n==-1&&m==-1) break;
        for(int i=0;i<m;i++)
        {
            scanf("%d%d",&s[i].a,&s[i].b);
            if(s[i].b!=0) s[i].x=1.0*s[i].a/s[i].b;
            else s[i].x=1000000;
        }
        sort(s,s+m);
        double ans=0;
        int tmp=n;
        int indx=0;
        while(indx<m)
        {
            if(tmp>=s[indx].b)
            {
                ans+=s[indx].a;
                tmp-=s[indx].b;
                indx++;
                //cout<<ans<<endl;
            }
            else
            {
                if(s[indx].x!=1000000) ans+=(1.0*s[indx].a*tmp/s[indx].b);
                else ans+=s[indx].a;
                tmp=0;
                break;
            }
        }
        printf("%.3f
",ans);
    }
    return 0;
}