HDOJ1050

/*重叠回合数最多，占用时间最长*/
#include <iostream>
#include <cstring>
#include <cmath>
#include <algorithm>
using namespace std;


#define M 1005


struct aa
{
double j;
double f;
double s;
}a[M];


bool cmp(aa a,aa b)  //bool型
{
return a.s > b.s;
}


int main()
{
double m;
int n;
while (cin >> m >> n, m != -1 && n != -1)
{
double v = 0;
for (int i = 0; i < n; i++)
{
cin >> a[i].j >> a[i].f;
a[i].s = a[i].j / a[i].f;
}
sort(a , a + n ,cmp); //自定规则
for (int i = 0; i < n; i++)
{
if (m == 0)break;
if (a[i].f <= m)
{
m -= a[i].f;
v += a[i].j;
}
else
{
v += m*a[i].s;
break;
}
}
printf("%.3lf\n", v);
}
return 0;
}