1.3 Mixing Milk

/*
ID:y7276571
LANG: C++
TASK: milk
*/
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#define MAXN 5000
typedef struct milk{
    int cost, ans;
}milk;
int cmp(const void *a, const void *b)
{
    return ((milk*)a)->cost - ((milk*)b)->cost;
}
int main(void)
{
    freopen("milk.in", "r", stdin);
    freopen("milk.out", "w", stdout);
    int need, n, i, cost = 0;
    milk mk[MAXN];
    memset(mk, 0, sizeof(mk));
    scanf("%d%d", &need, &n);
    for(i = 0; i < n; i++)  scanf("%d%d", &(mk[i].cost), &(mk[i].ans));
    qsort(mk, n, sizeof(milk), cmp);
    i = 0;
    while(need)
    {
        if(need > mk[i].ans) {need -= mk[i].ans; cost += mk[i].cost*mk[i++].ans; continue; }
        cost += need*mk[i].cost; break;
    }
    printf("%d\n", cost);
    return 0;
}