import sys

sys.setrecursionlimit(10 ** 6)
int1 = lambda x: int(x) - 1
p2D = lambda x: print(*x, sep="\n")
def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def SI(): return sys.stdin.readline()[:-1]

def main():
    def ok(c):
        if c==0:
            s=sum(w for i,w in xw)
            for a in aa:
                if a<s:return False
            return True
        cs=[0]*n
        for i,w in xw:
            i-=1
            k=w//c
            if k==0:
                cs[i]+=w
                if i+1<n:cs[i+1]-=w*2
                if i+2<n:cs[i+2]+=w
            else:
                si=max(i-k,0)
                cs[si]+=w-c*(i-si)
                cs[si+1]+=c-(w-c*(i-si))
                if i+1<n:cs[i+1]-=2*c
                if i+k+1<n:cs[i+k+1]+=c-(w-c*k)
                if i+k+2<n:cs[i+k+2]+=w-c*k
        for i in range(n-1):cs[i+1]+=cs[i]
        for i in range(n-1):cs[i+1]+=cs[i]
        for c,a in zip(cs,aa):
            if c>a:return False
        return True

    n,m=MI()
    aa=LI()
    xw=LLI(m)
    l=-1
    r=inf=10**9+5
    while l+1<r:
        c=(l+r)//2
        if ok(c):r=c
        else:l=c
    if r==inf:print(-1)
    else:print(r)

main()