Section 1.3 milk

标准的贪心，先将供应商按照价格的升序排序，然后按需购买价格低的牛奶就可以得到最小的总价。

/*
ID:yucept21
LANG:C++
TASK:milk
*/

#include<cstdio>
#include<cstdlib>
#include<cstring>

typedef struct tt
{
    int price, amount;
}T;

int N, M;
T milk[5005];
int cmp( const void *_p, const void *_q)
{
    T *p = (T *)_p;
    T *q = (T *)_q;
    return p->price - q->price;
}

int main()
{
    freopen( "milk.in", "r", stdin);
    freopen( "milk.out", "w", stdout);
    scanf( "%d%d", &N, &M);
    for( int i = 0; i < M; i++)
        scanf( "%d%d", &milk[i].price, &milk[i].amount);
    qsort( milk, M, sizeof( milk[0]), cmp);
    int ans = 0;
    for( int i = 0; i < M; i ++)
    {
        if( N >= milk[i].amount) {
            ans += milk[i].price * milk[i].amount;
            N -= milk[i].amount;
        }

        if( N < milk[i].amount) {
            ans += N * milk[i].price;
            break;
        }
    }
    printf( "%d\n", ans);
    return 0;
}