n, m = map(int, input().split())
a = list(map(int, input().split()))

u = [0 for i in range(m)]
v = [0 for i in range(m)]

for i in range(m) :
    u[i], v[i] = map(int, input().split())
    u[i] = u[i] - 1
    v[i] = v[i] - 1

ans = 0
while 1 :
    flag = 0
    for i in range(n) :
        if a[i] != 0 :
            flag = 1
            break
    if flag == 0 :
        break
    b = [0 for i in range(n)]
    for i in range(n) :
        b[i] = a[i]
        a[i] = a[i] // 2
    for i in range(m) :
        a[v[i]] += b[u[i]]
    ans = ans + 1

print(ans)