## https://yukicoder.me/problems/no/1008

def solve(N, M, A, xw, c):
    array = [0] * (N + 2)
    for x, w in xw:
        d = w // c
        l = max(0, x - d)
        r = min(N - 1, x + d)
        array[l + 1] += c
        array[x + 1] -= c
        array[x + 1] -= c
        array[r] += c
    
    cum_x = 0
    for i in range(N + 2):
        cum_x += array[i]
        array[i] = cum_x
    
    cum_x = 0
    for i in range(N + 2):
        cum_x += array[i]
        array[i] = cum_x
    for i in range(N):
        if array[i] >= A[i]:
            return False
    return True



def main():
    N, M = map(int, input().split())
    A = list(map(int, input().split()))

    xw = []
    sum_w = 0
    for _ in range(M):
        x, w = map(int, input().split())
        xw.append((x - 1, w))
        sum_w += w

    # そもそもcが=sum_wでも壊れることはない?
    cost = [0] * N
    for x, w in xw:
        cost[x] += w
    for i in range(N):
        if cost[i] >= A[i]:
            print(-1)
            return
        
    # cの壊れる最小値を2部探索で計算
    low = 0
    high = sum_w
    while high - low > 1:
        mid = (high + low) // 2
        if solve(N, M, A, xw, mid):
            high = mid
        else:
            low = mid
    if solve(N, M, A, xw, low):
        print(low)
    else:
        print(high)





if __name__ == "__main__":
    main()