N,M = map(int, input().split())
A = [int(a) for a in input().split()]
X = [0]*M
W = [0]*M
for i in range(M):
    x,w = map(int, input().split())
    X[i] = x
    W[i] = w

s = sum(W)
L = [0]*N
for i in range(M):
    L[X[i]-1] += W[i]
flag = True
for i in range(N):
    if A[i] < L[i]:
        flag = False
        break
if flag:
    sub = [0]*N
    for i in range(N):
        sub[i] = i*(i+1)//2 + (N-i-1)*(N-i)//2
    l = 0
    r = 10**9
    for _ in range(100):
        f = True
        m = (l+r)/2
        for i in range(N):
            if A[i] < s-sub[i]*m:
                f = False
                break
        if f:
            r = m
        else:
            l = m
    ans = int(l)+1
    if r < 1:
        ans = 0
else:
    ans = -1

print(ans)