见代码中的注释：

#include<bits/stdc++.h>
#define int long long
using namespace std;
const int mod = 1e9+7;
int n,s,l[200005],r[200005],pl,x,y,p,t[200005],cnt;
bool check(int x){
    p = 0,cnt = 0;
    for(int i = 1;i<=n;i++){
        if(r[i]>=x)t[++p] = max(x-l[i],0LL);
    }
    if(p<n/2+1)return 0;
    sort(t+1,t+p+1);
    for(int i = 1;i<=n/2+1;i++)cnt+=t[i];
    return cnt<=s-pl;
}
int find(){
    int l = 0,r = 1e18;
    while(l<r){
        int mid = (l+r)/2;
        if(check(mid))l = mid+1;
        else r = mid;
    }
    //return r;
    return r-1;//这里为什么要减一？
}
signed main(){
    int t;
    cin >> t;
    while(t--){
        pl = 0;
        cin >> n >> s;
        for(int i = 1;i<=n;i++){
            cin >> l[i] >> r[i];
            pl+=l[i];
        }
        cout << find() << "\n";
    }
	return 0;
}