蒟蒻调了半天没调出来。

#include <iostream>
#include <cctype>
#include <algorithm>
#define ll long long

using namespace std;

ll read()
{
    ll x = 0, f = 1;
    char ch = 0;
    while (!isdigit(ch))
    {
        if (ch == '-')
            f = -1;
        ch = getchar();
    }
    while (isdigit(ch))
    {
        x = x * 10 + ch - '0';
        ch = getchar();
    }
    return x * f;
}

const int maxN = 5e5 + 10;

ll T;
ll n, m;
ll ans;

struct node
{
    ll a, b, c;
} p[maxN];

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

int main()
{
    T = read();
    while (T--)
    {
        ans = 0;
        n = read(), m = read();
        for (int i = 1; i <= n; i++)
        {
            p[i].a = read(), p[i].b = read();
            p[i].c = p[i].b - p[i].a;
        }
        sort(p + 1, p + 1 + n, cmp);
        ll now = 1;
        while (now <= m - n && p[now].c > 0 && now <= n)
            ans += p[now++].b;
        if (now == n)
        {
            if (p[now - 1].a + p[now].a < p[now - 1].b + p[now].b)
                ans += p[now].b;
            else
                ans = ans - p[now - 1].b + p[now - 1].a + p[now].a;
        }
        else if (now < n)
            for (int i = now; i <= n; i++)
                ans += p[i].a;
        printf("%lld\n", ans);
    }
    return 0;
}