def Bisect_Int(ok,ng,is_ok):
    while abs(ok-ng)>1:
        mid=(ok+ng)//2
        if is_ok(mid):
            ok=mid
        else:
            ng=mid
    return ok

N,M=map(int,input().split())
A=list(map(int,input().split()))
X,W=[],[]
for m in range(M):
    x,w=map(int,input().split())
    x-=1
    X.append(x)
    W.append(w)
def is_ok(c):
    if c==0:
        sum_W=sum(W)
        return all(sum_W<a for a in A)
    else:
        def make_weight(X,W):
            imos=[0]*(N+2)
            for x,w in zip(X,W):
                if x-w//c>=0:
                    imos[x-w//c]+=w%c
                    imos[x-w//c+1]-=w%c
                    imos[x-w//c+1]+=c
                    imos[x+1]-=c
                    imos[x+1]+=-w
                    imos[x+2]-=-w
                else:
                    imos[0]+=w-x*c
                    imos[1]-=w-x*c
                    imos[1]+=c
                    imos[x+1]-=c
                    imos[x+1]+=-w
                    imos[x+2]-=-w
            for _ in range(2):
                for i in range(1,N+2):
                    imos[i]+=imos[i-1]
            return imos[:N]
        L=make_weight(X,W)
        R=make_weight([N-1-x for x in X],W)[::-1]
        LR=[l+r for l,r in zip(L,R)]
        for x,w in zip(X,W):
            LR[x]-=w
        return all(w<a for a,w in zip(A,LR))

inf=1<<20
ans=Bisect_Int(inf,-1,is_ok)
if ans==inf:
    ans=-1
print(ans)