[水题AC乐] - 贪心

HDU - 1009

#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<string>
#include<vector>
#include<stack>
#include<queue>
#include<set>
#include<map>
#define rep(i,j,k) for(register int i=j;i<=k;i++)
#define rrep(i,j,k) for(register int i=j;i>=k;i--)
#define erep(i,u) for(register int i=head[u];~i;i=nxt[i])
#define iin(a) scanf("%d",&a)
#define lin(a) scanf("%lld",&a)
#define din(a) scanf("%lf",&a)
#define s0(a) scanf("%s",a)
#define s1(a) scanf("%s",a+1)
#define print(a) printf("%lld",(ll)a)
#define enter putchar('\n')
#define blank putchar(' ')
#define println(a) printf("%lld\n",(ll)a)
#define IOS ios::sync_with_stdio(0)
using namespace std;
const int MAXN = 1e5+11;
const int INF = 0x3f3f3f3f;
const double EPS = 1e-7;
typedef long long ll;
const ll MOD = 1e9+7; 
ll read(){
    ll x=0,f=1;register char ch=getchar();
    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
    return x*f;
}
struct Node{
    ll j,f;
}a[MAXN];
int n,m;
bool cmp(Node a,Node b){
    return a.j*b.f>b.j*a.f;
}
int main(){
    while(cin>>m>>n){
        if(m==-1&&n==-1)break;
        rep(i,1,n){
            a[i].j=read();
            a[i].f=read();
        }
        sort(a+1,a+1+n,cmp);
        double ans=0;
        rep(i,1,n){
            if(m>a[i].f){
                m-=a[i].f;
                ans+=a[i].j;
            }else{
                ans+=1.0*m/a[i].f*a[i].j;
                break;
            }
        }
        printf("%.3lf\n",ans);
    }
    return 0;
}