CodeForces 16B Burglar and Matches(贪心)

B. Burglar and Matches

time limit per test

0.5 second

memory limit per test

64 megabytes

input

standard input

output

standard output

A burglar got into a matches warehouse and wants to steal as many matches as possible. In the warehouse there are mcontainers, in the i-th container there are ai matchboxes, and each matchbox contains bi matches. All the matchboxes are of the same size. The burglar's rucksack can hold n matchboxes exactly. Your task is to find out the maximum amount of matches that a burglar can carry away. He has no time to rearrange matches in the matchboxes, that's why he just chooses not more than n matchboxes so that the total amount of matches in them is maximal.

Input

The first line of the input contains integer n (1 ≤ n ≤ 2·108) and integer m (1 ≤ m ≤ 20). The i + 1-th line contains a pair of numbers ai and bi (1 ≤ ai ≤ 108, 1 ≤ bi ≤ 10). All the input numbers are integer.

Output

Output the only number — answer to the problem.

Sample test(s)

input

7 3
5 10
2 5
3 6

output

62

input

3 3
1 3
2 2
3 1

output

7

非常水的贪心  直接每次选装的多的盒子  n减去盒子数即可了

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
const int M = 25;
int a[M], b[M], c[M], n, m, ans;

bool cmp (int i, int j)
{
    return b[i] > b[j];
}

int main()
{
    scanf ("%d%d", &n, &m);
    for (int i = 1; i <= m; ++i)
    {
        c[i] = i;
        scanf ("%d%d", &a[i], &b[i]);
    }
    sort (c + 1, c + m + 1, cmp);
    ans = 0;
    for (int i = 1; i <= m; ++i)
    {
        if (n >= a[c[i]])
        {
            n -= a[c[i]];
            ans += a[c[i]] * b[c[i]];
        }
        else
        {
            ans += n * b[c[i]];
            break;
        }
    }
    printf ("%d\n", ans);
    return 0;
}