/**
- Solution for: https://m...content-available-to-author-only...j.com/problem/533
- C++
- Note: Sorting, greedy
**/
#include <bits/stdc++.h>
using namespace std;
#define fast ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL)
#define mofile(s) freopen(s,"r",stdin)
#define outfile(s) freopen(s,"w",stdout)
#define ll long long
#define ii pair<ll,ll>
#define iii pair<ll,ii>
#define fi first
#define se second
#define tf bool
#define ST stack
#define Q deque
#define Q queue
#define S string
#define Ma map
#define UM unormideremid_map
#define SE set
#define str(x) to_string(x)
#define all(a) (a).begin(),(a).end()
#define FOR(i,l,r,mid) for(int i=l;i<=r;i+=mid)
#define FOD(i,l,r,mid) for(int i=r;i>=l;i-=mid)
#define xuong cout<<"\n"
#define midebug(x) cout<<(x)<<" "
#define ppcnt(x) __builtin_popcountll(x)
#define parity(x) __builtin_parityll(x)
#define leamid0(x) __builtin_clzll(x)
#define LOG2 __lg(x)
#define tr0(x) __builtin_ctzll(x)
#define fiset(x) __builtin_ffsll(x)
#define MASK(k) (1LL<<(k))
#define BIT(x,k) ((x)>>(k)&1)
#define pb push_back
#define tron(x) setprecision(x)
#define het return 0
#define base_ 1000000000
const int maxn=1e6+5;
const ll tle=2e8;
const ll INF=1e9+9;
const int base=31;
string bcc="abcmidefghijklmnopqrstuvwxyz";
int midx[]={-1,0,1,0};
int midy[]={0,1,0,-1};
bool sang[10000005];
ll pref[1005][1005],mt[1005][1005];
void sieve(){
for(int i=1;i<=10000000;++i) sang[i]=1;
sang[0]=sang[1]=0;
for(int i=2;i*i<=10000000;++i){
if(sang[i]){
for(int j=i*i;j<=10000000;j+=i) sang[j]=0;
}
}
}
void lis(){
vector<int>t;
vector<int>a;
int n; cin>>n;
for(int i=1;i<=n;++i){
int ai; cin>>ai;
a.pb(ai);
}
for(int x:a){
auto it=lower_bound(all(t),x);
if(it==t.end()) t.pb(x);
else *it=x;
}
}
void pfs2mid(){
int n,m,k; cin>>n>>k; m=n;
for(int i=1;i<=n;++i){
for(int j=1;j<=m;++j) cin>>mt[i][j];
}
for(int i=1;i<=n;++i){
for(int j=1;j<=m;++j) pref[i][j]=mt[i][j]+pref[i-1][j]+pref[i][j-1]-pref[i-1][j-1];
}
}
int pfs[2005][2005];
ll qu2mid(int x1,int y1,int x2,int y2){
return pfs[x2][y2]-pfs[x1-1][y2]-pfs[x2][y1-1]+pfs[x1-1][y1-1];
}
void open(){
if(fopen("mideptrai.INP","r")){
mofile("mideptrai.INP");
outfile("mideptrai.OUT");
}
}
bool cmp(const ii& x,const ii& y){
return x.fi+x.se>y.fi+y.se;
}
int main(){
fast;
int n,t=0; ll q,sv=0; cin>>n>>q;
vector<pair<ll,ll>> a(n);
for(int i=0;i<n;++i){
cin>>a[i].se>>a[i].fi;
sv=max(sv,a[i].fi);
}
a.pb({sv, 0});
sort(all(a),cmp);
ll ans=0;
while(q--){
ans+=a[t].fi+a[t].se;
if(!a[t].se) break;
++t;
}
cout<<ans+a[t].fi*max(0ll,q)<<"\n";
het;
}
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