#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);
}

bool compareVal(pair<int,int>a,pair<int,int>b){
    return a.fi+a.se>b.fi+b.se;
}

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

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]);

        sum.insert({s[i]+e[i],i});
    }

    while(q--){
        pair<int,int> best=*sum.rbegin();
        if(marked[best.se]==false){
            marked[best.se]=true;
            sum.erase(sum.find(best));
            sum.insert({best.fi-e[best.se],best.se});
        }
        else{
            break;
        }
        ans+=best.fi;
    }


    cout<<ans+mx_tmp*(q+1);
    return 0;
}