C++ :

#include<iostream>
#include<algorithm>
using namespace std;
const int N = 1005;
struct food{
	int w,v;
};
food a[N];

bool comp(food x,food y){
	return x.v>y.v;
}

int main(){
	int M,n,ans=0;
	cin>>M>>n;
	for (int i=1; i<=n; i++) cin>>a[i].w>>a[i].v;
	sort(a+1,a+n+1,comp);
	for (int i=1; i<=n&&M>0; i++)
		if (M>=a[i].w) ans+=a[i].v*a[i].w,M-=a[i].w;
		else ans+=a[i].v*M,M=0;
	cout<<ans<<endl;
	return 0;
}