#include <cstring>
#include <iostream>
#include <queue>

using namespace std;
const int N = 10005;
const int M = 20005;

int v[N], n, m, a, b, cnt, fr, to, dis[N], cp[N];
bool vis[N];
struct {
    int next, to, v;
} edge[M];
int head[N];
queue<int> q;
void addEdge(int u, int v, int w)
{
    edge[++cnt].next = head[u];
    edge[cnt].to = v;
    edge[cnt].v = w;
    head[u] = cnt;
}

int SPFA()
{
    memset(dis, 0x3f, sizeof(dis));
    dis[a] = 0;
    q.push(a);
    while (!q.empty()) {
        int u = q.front();
        q.pop();
        vis[u] = 0;
        for (int to = head[u]; to; to = edge[to].next) {
            int v = edge[to].to;
            int w = edge[to].v;
            if (dis[v] > dis[u] + w) {
                dis[v] = dis[u] + w;
                cp[v] = cp[u] + 1;
                if (cp[v] >= n)
                    return false;
                if (!vis[v])
                    q.push(v), vis[v] = true;
            }
        }
    }
    return true;
}

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> n >> m >> a >> b;
    for (int i = 1; i <= n; i++)
        cin >> v[i];
    while (m--) {
        cin >> fr >> to;
        addEdge(fr, to, v[to + 1] - v[fr + 1] + 1);
    }
    if (SPFA() && dis[b] != 0x3f3f3f3f)
        cout << dis[b];
    else
        cout << "No solution";
    return 0;
}