def main():
    import sys
    input = sys.stdin.readline

    N, M = map(int, input().split())
    A = list(map(int, input().split()))
    weight = [0] * N
    for _ in range(M):
        x, w = map(int, input().split())
        weight[x-1] = w

    for i in range(N):
        if A[i] <= weight[i]:
            print(-1)
            exit()

    if sum(weight) < min(A):
        print(0)
        exit()

    ok = 10**9
    ng = 0
    mid = (ok+ng)//2
    while ok - ng > 1:
        B = [0] * N
        imos1 = [0] * N
        imos2 = [0] * N
        for x, w in enumerate(weight):
            if w:
                L, mod = divmod(w, mid)
                imos1[max(0, x-L)] += mod
                if x+L+1 < N:
                    imos1[x+L+1] -= mod
                if L:
                    if x-L+1 >= 0:
                        imos2[x-L+1] += 1
                    else:
                        imos2[0] += 1
                        imos1[0] += L-x-1
                    if x + 1 < N:
                        imos2[x + 1] -= 2
                    if x + L < N:
                        imos2[x + L] += 1
        for i in range(1, N):
            imos2[i] += imos2[i-1]
        B[0] = imos1[0] + imos2[0] * mid
        for i in range(1, N-1):
            B[i] = B[i-1] + imos1[i] + imos2[i] * mid
        flg = 1
        for a, b in zip(A, B):
            if b >= a:
                flg = 0
                break
        if flg:
            ok = mid
        else:
            ng = mid
        mid = (ok+ng)//2
        #print(ok, ng, mid)
    print(ok)


if __name__ == '__main__':
    main()