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| #include <bits/stdc++.h>
using namespace std;
const double eps = 1e-10;
const double pi = 3.1415926535897932384626433832795;
const double eln = 2.718281828459045235360287471352;
#define f(i, a, b) for (int i = a; i <= b; i++)
#define scan(x) scanf("%d", &x)
#define mp make_pair
#define pb push_back
#define lowbit(x) (x&(-x))
#define fi first
#define se second
#define SZ(x) int((x).size())
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define summ(a) (accumulate(all(a), 0ll))
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef vector<int> vi;
using ll=long long;
ll n,m,k,d,s[(int)3e5+9],now[(int)3e5+9],dep[(int)3e5+9],dis[(int)3e5+9],cost[(int)3e5+9],qsb[(int)3e5+9],ans=1e18;
vector<int> g[(int)1e5+9];
bitset<(int)3e5+9> vis;
int main()
{
ios::sync_with_stdio(false);
cin.tie(0);
cin>>n>>m>>k>>d;
for(int i=1,x,y;i<=m;++i){
cin>>x>>y;
g[x].push_back(y),g[y].push_back(x);
}
for(int i=1;i<=m;++i){
s[i]=s[i-1]+d-(i-1)%d;
}
for(int i=1;i<=d;++i){
now[i]=now[i-1]+i;
}
{
queue<int> q;
q.push(n);
dis[n]=1;
while(!q.empty()){
auto p=q.front();q.pop();
for(auto it:g[p])if(!dis[it])dis[it]=dis[p]+1,q.push(it);
}
}
{
queue<int> q;
q.push(k);
dep[k]=d;
while(!q.empty()){
auto p=q.front();q.pop();
for(auto it: g[p]){
if(!dep[it]){
dep[it]=dep[p]-1;
if(dep[it])q.push(it);
}
}
}
}
{
priority_queue<pair<ll,int>,vector<pair<ll,int>>,greater<pair<ll,int>>>q;
q.push({0,1});
memset(cost,0x3f,sizeof(cost));
cost[1]=0ll;
while(!q.empty()){
auto [aa,bb]=q.top();q.pop();
if(vis[bb])continue;
vis[bb]=1;
for(auto it:g[bb]){
if(dep[it]){
if(cost[it]>cost[bb]+dep[it]){
cost[it]=cost[bb]+dep[it];
q.push({cost[it],it});
}
}else if(!vis[it]) ans=min(ans,cost[bb]+s[dis[it]]);
}
}
}
ans=min(ans,cost[n]);
cout<<ans;
return 0;
}
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