#include<bits/stdc++.h>
using namespace std ;
typedef long long ll ;
typedef unsigned long long ull ;
typedef pair < int , int > pii ;
typedef vector < int > vi ;
#define fi first
#define se second
mt19937 rng(chrono::high_resolution_clock::now().time_since_epoch().count());

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()

const int MOD = 998244353 ;
const int MAXN = 1e4 + 7 ;

int n , m ;
vector < int > v[ MAXN ] ;

ll a[ MAXN ] , b[ MAXN ] ;

void solve ( ) {
    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 ;
        v[ x ].push_back ( y ) ;
    }
    int ans = 0 ;
    while ( 1 ) {
        bool done = true ;
        for ( int i = 1 ; i <= n ; ++ i ) {
            if ( a[ i ] > 0 ) {
                done = false ;
                break ;
            }
        }
        for ( int i = 1 ; i <= n ; ++ i ) {
            b[ i ] = a[ i ] ;
            a[ i ] = 0 ;
        }
        if ( done == true ) { break ; }
        ++ ans ;
        for ( int i = 1 ; i <= n ; ++ i ) {
            for ( auto j : v[ i ] ) {
                a[ j ] += b[ i ] ;
            }
            a[ i ] += b[ i ] / 2 ;
        }
    }
    cout << ans << "\n" ;
}

int main ( ) {
    ios_base :: sync_with_stdio ( false ) ;
    cin.tie ( NULL ) ;
    int t = 1 ; // cin >> t ; 
    while ( t -- ) { solve ( ) ; }
    return 0 ;
}