关键字重排序

#include <iostream>
#include <cstring>
#include <algorithm>

#define x first
#define y second

using namespace std;

typedef long long LL;
typedef pair<int, int> PII;

const int N = 2e6 + 10;

PII a[N];
int n, m;

int main()
{
    scanf("%d%d", &m, &n);

    for(int i = 0; i < n; i ++) scanf("%d%d", &a[i].x, &a[i].y);
    sort(a, a + n);

    LL res = 0;
    for(int i = 0; i < n; i ++)
    {
        if(m >= a[i].y) res += (LL)a[i].y * a[i].x, m -= a[i].y;
        else
        {
            res += (LL)m * a[i].x;
            break;
        }
    }

    printf("%lld", res);

    return 0;
}