#include<bits/stdc++.h>
using namespace std;
int n,m;
int a[100100];
int deg[100100];
int f[1010][1010];
queue<int>q;
int ans;
vector<int>g[100100];
int main(){
    cin>>n>>m;
    for (int i=1;i<=n;i++)
    cin>>a[i];
    for (int i=1,x,y;i<=m;i++)
    cin>>x>>y,g[x].push_back(y),
    deg[y]++;
    for (int i=1;i<=n;i++)
    if(deg[i]==0)q.push(i);
    int nn=n+30;
    for (int i=1;i<=n;i++){
        for (int j=0;j<=30;j++)
        if(a[i]>>j&1)f[i][j]=1,ans=max(ans,j+1);
    }
    while(q.size()){
        int x=q.front();q.pop();
        for (int i=0;i<=nn;i++)
        if(f[x][i]==1)ans=max(ans,i+1);
        for (auto y:g[x]){
            for (int i=1;i<=nn;i++){
                f[y][i]+=f[x][i-1];
                if(f[y][i]>1)f[y][i+1]+=f[y][i]/2,f[y][i]&=1;
            }
            deg[y]--;
            if(deg[y]==0)q.push(y);
        }
    }
    cout<<ans<<"\n";
}