编辑代码

#include <bits/stdc++.h>
using namespace std;

int n,m;//n--需奶量
int p,a;
long long sum;
int m1;

map <int,int>mp;

int main() {

    cin>>n>>m;

    for(int i = 1;i <= m;i++)
    {
        cin>>p>>m1;
        mp[p] += m1;
    }

    for(auto x:mp)
    {
        if(n >= x.second)
        {
            sum += x.first * x.second;
            n -= x.second;
        }
        else
        {
            sum += x.first * n;
            break;
        }
    }

    cout<<sum<<endl;

	return 0;
}