#include<bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
typedef long long int ll;
typedef vector<ll> vi;
typedef vector<vector<ll>> vvi;
typedef pair<ll,ll> pi;
typedef map<ll,ll> mi;
typedef long double ld;
typedef vector<ld> vd;
typedef vector<vector<ld>> vvd;
typedef pair<ld,ld> pd;
#define ff first
#define ss second
#define srt(a) sort(a.begin(),a.end());
#define fip(k, n) for (ll i = k; i < n; i++)
#define fjp(k, n) for (ll j = k; j < n; j++)
#define fin(k, n) for (ll i = k; i >= n; i--)
#define fjn(k, n) for (ll j = k; j >= n; j--)
#define fp(k, n, m) for (ll k = n; k < m; k++)
#define fn(k, n, m) for (ll k = n; k >= m; k--)
#define ordered_set tree<pi, null_type,less< pi >, rb_tree_tag,tree_order_statistics_node_update>
#define totalOne(n) __builtin_popcountll(n)
#define backZero(n) __builtin_ctzll(n)
#define frontZero(n) __builtin_clzll(n)
#define fx(k) for ( auto x : k )
#define test ll t;cin >> t;while (t--)
#define nli "\n"
// ==========================(debug)============================================================================================== //
#ifndef ONLINE_JUDGE
#define debug(x) cerr << #x <<" "; _printn(x); cerr << nli;
#else
#define debug(x)
#endif
void _printn(ll x){ cerr<<x<<" "; }
void _printn(int x){ cerr<<x<<" "; }
void _printn(ld x){ cerr<<x<<" "; }
void _printn(double x){ cerr<<x<<" "; }
void _printn(string x){ cerr<<x<<" "; }
void _printn(char x){ cerr<<x<<" "; }
void _printn(bool x){ cerr<<x<<" "; }
template<class T,class V>void _printn(pair<T,V> vv);
template<class T> void _printn(vector<T> vv);
template<class T> void _printn(set<T> vv);
template<class T,class V> void _printn(map<T,V> vv);
template<class T> void _printn(multiset<T> vv);
template<class T,class V>void _printn(pair<T,V> vv){ cerr<<"( ";_printn(vv.ff);cerr<<",";_printn(vv.ss);cerr<<")";}
template<class T> void _printn(vector<T> vv){ cerr<<"[ "; for(auto xx:vv){ _printn(xx);cerr<<" "; } cerr<<"]"; };
template<class T> void _printn(set<T> vv){ cerr<<"{ "; for(auto xx:vv){ _printn(xx);cerr<<" "; } cerr<<"}"; };
template<class T> void _printn(multiset<T> vv){ cerr<<"{ "; for(auto xx:vv){ _printn(xx);cerr<<" "; } cerr<<"}"; };
template<class T,class V> void _printn(map<T,V> vv){ cerr<<"{ "; for(auto xx:vv){ _printn(xx);cerr<<" "; } cerr<<"}"; };
// ==========================(debug)============================================================================================== //
ll n,m,tp,tp2,res,cnt,sum,tptp,ans;
const ll mx = 10+1;
const ll mod = 1e9+7;
// ==========================(MOD)=============================================================================================== //
ll mod_add(ll aa,ll bb){ return ((aa%mod)+(bb%mod))%mod; }
ll mod_minus(ll aa,ll bb){ return (((aa%mod)-(bb%mod))+10*mod)%mod; }
ll mod_mul(ll aa,ll bb){ return ((aa%mod)*(bb%mod))%mod; }
ll mod_power(ll aa,ll bb){ aa%=mod; ll empowered = 1; bb%=mod-1; while(bb > 0){ if(bb & 1) empowered = mod_mul(empowered,aa); bb = bb >> 1; aa = mod_mul(aa,aa); } return empowered; }
ll mod_divi(ll aa,ll bb){ aa=mod_mul(aa,mod_power(bb,mod-2)); return aa; }
// ==========================(MOD)=============================================================================================== //
bool f = false;
const ll mxn = 20 + 3;
ll a[mx];
ll b[mx];
ll dp[mxn][1<<mx][mx];
int main(){
ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
// #ifndef ONLINE_JUDGE
// freopen("input1.txt", "r", stdin);
// freopen("output1.txt", "w", stdout);
// freopen("error1.txt", "w", stderr);
// #endif // ONLINE_JUDGE
cin>>n>>m;
ans= 0;
fip(0,n)
cin>>a[i]>>b[i];
fip(0,mxn)
fjp(0,1<<mx)
fp(k,0,mx)
dp[i][j][k] = 1e18;
fip(0,n)
dp[a[i]][1<<i][1] = 0;
tptp = 1 << n;
tptp--;
ll sm = 0,pt;
fip(0,21){
fjp(0,(1<<n)){
res = tptp - j;
cnt = 1e18;
fp(k,0,n)
if((j & (1<<k)))
cnt = min(cnt,b[k]);
// debug(i);
// debug(j);
// bitset<5> kk(j);
// debug(dp[i][j][3]);
// cerr<<kk<<nli;
// debug(cnt);
while(res > 0){
f = 0;
sum = 0;
fp(k,0,n){
if((res & (1<<k)) && a[k] > i+1)
f = 1;
else if((res & (1<<k))){
sum += (i+1-a[k]) * m;
}
}
if(f){
res = (res-1) & (tptp-j);
continue;
};
// debug(res);
// debug(sum);
bitset<5> kk(res);
// cerr<<kk<<nli;
fp(k,0,n+1){
pt = min((ll)totalOne(res),k);
sm = sum;
tp2 = totalOne(res) - pt;
// debug(k);
sm += (tp2*cnt);
// debug(dp[i][j][3]);
// debug(sm);
dp[i+1][res|j][k-pt+totalOne(res)] = min(dp[i][j][k]+sm,dp[i+1][res|j][k-pt+totalOne(res)]);
// debug(dp[i+1][res|j][k-pt+totalOne(res)]);
// cerr<<nli;
}
res = (res-1) & (tptp-j);
}
}
}
ans = 1e18;
fip(0,21)
fp(k,0,n+1)
ans = min(ans,dp[i][tptp][k]);
cout<<ans<<nli;
cerr << "Time elapsed: " << setprecision(6) << 1000.0 * clock() / CLOCKS_PER_SEC << "ms\n";
return 0;
}
