#include <bits/stdc++.h>

#define LL long long
#define ull unsigned long long
#define F(i, j, k) for (int i = j; i <= k; ++i)
#define DF(i, j, k) for (int i = j; i >= k; --i)

using namespace std;

template <typename T> inline void read(T &n) {
    T w = 1;
    n = 0;
    char ch = getchar();
    while (!isdigit(ch) && ch != EOF) {
        if (ch == '-') w = -1;
        ch = getchar();
    }
    while (isdigit(ch) && ch != EOF) {
        n = (n << 3) + (n << 1) + (ch & 15);
        ch = getchar();
    }
    n *= w;
}

template <typename T> inline void write(T x) {
    T l = 0;
    ull y = 0;
    if (!x) { putchar(48); return; }
    if (x < 0) { x = -x; putchar('-'); }
    while (x) { y = y * 10 + x % 10; x /= 10; ++l; }
    while (l) { putchar(y % 10 + 48); y /= 10; --l; }
}

template <typename T> inline void writes(T x) {
    write(x);
    putchar(' ');
}

template <typename T> inline void writeln(T x) {
    write(x);
    puts("");
}

template <typename T> inline void checkmax(T &a, T b) { a = a > b ? a : b; }

template <typename T> inline void checkmin(T &a, T b) { a = a < b ? a : b; }

const int N = 1e4 + 100, M = 1e5 + 100;

int mx = 500;

LL a[N][505];

int cnt, d[N], head[N], to[M], nxt[M];

inline void addedge(int x, int y) {
    nxt[++cnt] = head[x];
    to[cnt] = y;
    head[x] = cnt;
}

queue <int> q;

const LL MOD = 1LL << 63;

inline void add(int x, int y) {
    F(i, 0, mx) {
        a[y][i] += a[x][i] * 2;
        a[y][i + 1] += a[y][i] / MOD;
        a[y][i] %= MOD;
    }
}

inline int solve(LL now) {
    int ans = 0;
    while (now) {
        now >>= 1;
        ++ans;
    }
    return ans;
}

int main() {
    //freopen(".in", "r", stdin);
    //freopen(".out", "w", stdout);
    int n, m;
    read(n); read(m);
    F(i, 1, n) read(a[i][0]);
    F(i, 1, m) {
        int x, y;
        read(x); read(y);
        addedge(x, y);
        ++d[y];
    }
    F(i, 1, n) if (!d[i]) q.push(i);
    int las;
    while (!q.empty()) {
        int x = q.front(); q.pop();
        las = x;
        for (int i = head[x]; i; i = nxt[i]) {
            add(x, to[i]);
            --d[to[i]];
            if (!d[to[i]]) q.push(to[i]);
        }
    }
    int s = -1;
    DF(i, mx, 0) if (a[las][i]) { s = i; break; }
    if (s == -1) { puts("0"); return 0; }
    writeln(63 * s + solve(a[las][s]));
    return 0;
}