#include <bits/stdc++.h>

using namespace std;
bool sortWay(const pair<unsigned long long,unsigned long long> &a,
              const pair<unsigned long long,unsigned long long> &b)
{
    return (min(a.first * 2, a.second) - a.first > min(b.first * 2, b.second) - b.first);
}
int main()
{
unsigned long long n, f, k, l, sum = 0;
vector<pair<unsigned long long, unsigned long long> > v;
cin >> n >> f;
while(n--){
    cin >> k >> l;
    v.push_back(make_pair(k, l));
    sum += min(k, l);
}
sort(v.begin(), v.end(), sortWay);
for(int i = 0; i < f; i++){
    unsigned long long first = v[i].first, second = v[i].second;
    sum +=  min(first * 2, l) - first;
}
cout << sum << endl;
    return 0;
}