#include <bits/stdc++.h>

#define pb push_back
#define fi first
#define se second

using namespace std;

typedef long long ll;
#define int ll

const __int128 base = __int128(1) << 120;
const int sz = 7e4 / 120;
typedef array<__int128, sz> superll;

void add(superll& x, const superll& y)
{
    __int128 carry = 0;
    for (int i=0; i<sz; i++)
    {
        x[i] = (x[i] + 2*y[i] + carry)%base;
        carry = (x[i] + 2*y[i] + carry)/base;
    }
}

vector<int> g[100500];
vector<int> gr[100500];
int a[10500];
bool used[10500];
superll dp[10500];

void dfs(int v)
{
    if (used[v])
        return;
    for (int i=1; i<sz; i++)
        dp[v][i] = 0;
    dp[v][0] = a[v];

    used[v] = true;
    for (int i : gr[v])
    {
        dfs(i);
        add(dp[v], dp[i]);
    }
}

signed main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

//    cout<<sz*16*10000 / 1e6<<"\n";
//    return 0;

    int n, m;
    cin>>n>>m;
    for(int i=1; i<=n; i++)
    {
        cin>>a[i];
    }
    for (int i=1; i<=m; i++)
    {
        int x, y;
        cin>>x>>y;
        g[x].pb(y);
        gr[y].pb(x);
    }

//    n = 1e4;
//    m = 1e5;
//    for (int i=1; i<=n; i++)
//        a[i] = 1e9;
//    for (int i=1; i<n; i++)
//    for (int j=1; j<=m/n; j++)
//    {
//        int x, y;
//        x = i;
//        y = i+1;
//        g[x].pb(y);
//        gr[y].pb(x);
//    }

    int x = 0;
    for (int i=1; i<=n; i++)
        if (g[i].size() == 0)
            x = i;
    dfs(x);
    int last = -1;
    for (int i=sz-1; i>=0; i--)
        if (dp[x][i])
    {
        last = i;
        break;
    }
    if (last == -1)
        cout<<"0\n";
    else
    {
//        cout<<last<<" "<<sz<<"\n";
//        cout<<ll(dp[x][last])<<"\n";
        int ans = last*120;
        __int128 y = dp[x][last];
        while (y > 0)
            ans++, y/=2;
        cout<<ans<<"\n";
    }
}
/*
3 2
1 1 1
1 2
2 3

6 8
1 1 4 5 1 4
1 4
1 5
2 3
2 5
3 4
4 5
4 6
5 6

5 6
7 2 3 6 6
1 2
1 4
2 3
3 4
3 5
4 5
*/