C++ :

#include<bits/stdc++.h>
using namespace std;
#define endl '\n'
#define fi first
#define se second
#define pii pair<int,int>
#define pb push_back
int n,m,a,b,v[100005],vis[100005];
vector<int>g[100005];
int main(){
	cin>>n>>m>>a>>b;
	for(int i=0;i<n;i++) cin>>v[i];
	while(m--){
		int x,y;
		cin>>x>>y;
		g[x].pb(y);//有向边
	}
    //广搜
	queue<pii>q;
	q.push({a,0});
	while(!q.empty()){
		int x=q.front().fi;
		int y=q.front().se;
		if(x==b){//到达终点
			cout<<y+v[b]-v[a];
			return 0;
		}
		q.pop();
		for(auto i:g[x]){
			if(!vis[i]){
				vis[i]=1;
				q.push({i,y+1});
			}
		}
	}
	cout<<"No solution";//无解
    return 0;
}