#include <bits/stdc++.h>

using namespace std;

int main(){
long long n,f;
cin >> n >> f;
vector<pair<long long,long long>> v;
for(long long i=0;i<n;i++)
{
  long long a,b;
cin >> a >> b;
v.push_back(make_pair(a,b));
}
sort(v.begin(),v.end());
for(long long i=n-1;i>=0;i--)
{
  if(f>0 and v[i].second>0)
  {
    v[i].first*=2;
    f--;
  }
}
long long count=0;
for(long long i=0;i<n;i++)
{
  count+=min(v[i].first,v[i].second);
}
cout << count;

    return 0;
}