import sys
input = sys.stdin.readline

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

PE=[tuple(map(int,input().split())) for i in range(M)]

P=[0]*(N+1)
for x,w in PE:
    P[x]+=w

for i in range(N):
    if A[i]<P[i+1]:
        print(-1)
        sys.exit()

OK=1<<62
NG=-1

while OK-NG>1:
    mid=(OK+NG)//2
    
    P=[0]*(N+2)
    R=[0]*(N+2)
    L=[0]*(N+2)

    RR=[0]*(N+2)
    LL=[0]*(N+2)
    
    for x,w in PE:
        if mid!=0:
            h=w//mid
        else:
            h=10**9

        P[max(1,x-h)]+=w
        if x+h+1<N+2:
            P[x+h+1]-=w

        if h==0:
            continue

        R[x+1]-=mid
        if x+h+1<N+2:
            R[x+h+1]+=mid
        if x+h+1<N+2:
            RR[x+h+1]+=h*mid

        L[x-1]-=mid
        if x-h-1>=0:
            L[x-h-1]+=mid
        if x-h-1>=0:
            LL[x-h-1]+=h*mid

    #print(P,R,L,RR,LL)

    for i in range(1,N+2):
        P[i]+=P[i-1]
        R[i]+=R[i-1]

    for i in range(N,-1,-1):
        L[i]+=L[i+1]

    for i in range(N+2):
        R[i]+=RR[i]
        L[i]+=LL[i]

    for i in range(1,N+2):
        R[i]+=R[i-1]

    for i in range(N,-1,-1):
        L[i]+=L[i+1]

    #print(mid,P,R,L)
    #print([P[i]+R[i]+L[i] for i in range(1,N+1)])

    for i in range(1,N+1):
        if P[i]+R[i]+L[i]<=A[i-1]:
            True
        else:
            NG=mid
            break
    else:
        OK=mid
print(OK)