#include<bits/stdc++.h>
#define int long long
#define endl '\n'
using namespace std;

const int N = 500010;

struct P{
    int a,b;
    int c;
}p[N];

bool cmp(P a,P b){
    return a.c>b.c;
}

void slove(){
    int n,m;
    cin>>n>>m;
    int sum=0;
    for(int i=0;i<n;i++){
        cin>>p[i].a>>p[i].b;
        sum+=p[i].a;
        p[i].c=p[i].b-p[i].a;
    }
    sort(p,p+n,cmp);

    if(n==1){
        cout<<p[0].b<<endl;
        return;
    }

    int ans=sum;
    int idx=0;
    int num=n;
    while(num<m){
        sum+=p[idx++].c;
        if(idx==n-1) sum+=p[idx].c;
        ans=max(ans,sum);
        num++;
        if(idx==n-1) break;
    }
    cout<<ans<<endl;
}
signed main(){
    ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);
    int _=1;
    cin>>_;
    while(_--){
        slove();
    }
}