HOJ-1003

 1 #include<cstdio>
 2 #include<algorithm>
 3 
 4 #define MAXM 5010
 5 
 6 struct point{
 7   int pri, amo;
 8   bool operator < (const point& b) const{
 9     return pri < b.pri;
10   }
11 };
12 
13 point a[MAXM];
14 
15 int main(void)
16 {
17   int N, M;
18   int ans = 0;
19   scanf("%d%d", &N, &M);
20   for(int i = 0; i < M; ++i) scanf("%d%d", &a[i].pri, &a[i].amo);
21   std::sort(a, a+M);
22   //for(int i = 0; i < M; ++i) printf("%d", a[i].pri);
23   for(int i = 0; i < M; ++i){
24     if(a[i].amo >= N){
25       ans += N * a[i].pri;
26       break;
27     }
28     else{
29       ans += a[i].amo * a[i].pri;
30       N -= a[i].amo;
31     }
32   }
33   printf("%d\n", ans);
34 
35   return 0;
36 }