#include <bits/stdc++.h>

using namespace std;

#define ll long long
#define int long long
#define pb push_back
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second

int const MAXN=1e6+10;
int const INF=1e18;

void open(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL); cout.tie(NULL);
//    freopen(TASK".INP","r",stdin);
//    freopen(TASK".OUT","w",stdout);
}

int s[MAXN],e[MAXN],a[MAXN];
int n,q,ans,mx_tmp;

signed main(){
    open();
    cin>>n>>q;

    multiset<pair<int,int>> sum;

    for(int i=1;i<=n;i++){
        cin>>e[i]>>s[i];
        mx_tmp=max(mx_tmp,s[i]);
        a[i]=s[i]+e[i];
    }

    sort(a+1,a+n+1);

    for(int i=n;i>=1;i--){
        if(q>0){
            ans+=max(mx_tmp,a[i]);
        }
        else break;
        q--;
    }

    if(q>0)ans+=mx_tmp*q;

    cout<<ans;
    return 0;
}