import sys
input = sys.stdin.readline

class LazySegmentTree:
    def __init__(
        self,
        n,
        identity_e_node,
        identity_e_lazy,
        combine_node_f,
        combine_lazy_f,
        reflect_f,
    ):
        self._n = n
        self._size = 1
        self._height = 0
        while self._size < self._n:
            self._size <<= 1
            self._height += 1
        self._identity_e_node = identity_e_node
        self._identity_e_lazy = identity_e_lazy
        self._combine_node_f = combine_node_f
        self._combine_lazy_f = combine_lazy_f
        self._reflect_f = reflect_f
        self._node = [self._identity_e_node] * (2 * self._size)
        self._lazy = [self._identity_e_lazy] * (2 * self._size)

    #遅延データの値を値データに反映させたときの値を返す。
    def _reflect_lazy(self, index):
        return self._reflect_f(self._node[index], self._lazy[index])

    def _propagate_from_top(self, index):
        index += self._size
        for h in range(self._height, 0, -1):
            i = index >> h
            if self._lazy[i] != self._identity_e_lazy:
                self._lazy[i << 1] = self._combine_lazy_f(
                    self._lazy[i << 1], self._lazy[i]
                )
                self._lazy[i << 1 | 1] = self._combine_lazy_f(
                    self._lazy[i << 1 | 1], self._lazy[i]
                )
                self._node[i] = self._reflect_lazy(i)
                self._lazy[i] = self._identity_e_lazy

    def _update_from_bottom(self, index):
        index = (index + self._size) >> 1
        while index:
            self._node[index] = self._combine_node_f(
                self._reflect_lazy(index << 1),
                self._reflect_lazy(index << 1 | 1)
            )
            index >>= 1

    def build(self, array):
        assert len(array) == self._n
        for index, value in enumerate(array, start=self._size):
            self._node[index] = value
        for index in range(self._size - 1, 0, -1):
            self._node[index] = self._combine_node_f(
                self._node[index << 1],
                self._node[index << 1 | 1]    
            )

    # 区間更新 位置[L, R) (0-indexed)を値valueで更新
    def update(self, L, R, value):
        self._propagate_from_top(L)
        self._propagate_from_top(R - 1)
        L_lazy = L + self._size
        R_lazy = R + self._size
        while L_lazy < R_lazy:
            if L_lazy & 1:
                self._lazy[L_lazy] = self._combine_lazy_f(self._lazy[L_lazy], value)
                L_lazy += 1
            if R_lazy & 1:
                R_lazy -= 1
                self._lazy[R_lazy] = self._combine_lazy_f(self._lazy[R_lazy], value)
            L_lazy >>= 1
            R_lazy >>= 1
        self._update_from_bottom(L)
        self._update_from_bottom(R - 1)

    # 区間取得 区間[L, R) (0-indexed)内の要素について
    # L番目から順にcombine_node_fを適用した値を返す。
    def fold(self, L, R):
        self._propagate_from_top(L)
        self._propagate_from_top(R - 1)
        L += self._size
        R += self._size
        value_L = self._identity_e_node
        value_R = self._identity_e_node
        while L < R:
            if L & 1:
                value_L = self._combine_node_f(value_L, self._reflect_lazy(L))
                L += 1
            if R & 1:
                R -= 1
                value_R = self._combine_node_f(self._reflect_lazy(R), value_R)
            L >>= 1
            R >>= 1
        return self._combine_node_f(value_L, value_R)

def g(v, s):
    return v * N + s

def g2(x):
    v, s = divmod(x, N)
    return (v, s)

def op(x1, x2):
    v1, s1 = g2(x1)
    v2, s2 = g2(x2)
    return g(v1 + v2, s1 + s2)

def reflect_f(node_, lazy):
    node = g2(node_)
    return g(node[0] + lazy * node[1], node[1])


N, M = map(int, input().split())
A = list(map(int, input().split()))
X, W = [0] * M, [0] * M
for i in range(M):
    X[i], W[i] = map(int, input().split())
    X[i] -= 1
from operator import add
    
    
def f(l, vl, r, vr, c, T):
    T.update(l+1, r+1, c)
    T.update(l, l + 1, vl)
    T.update(r+1, r + 2, -vr)
    return T
    
def printT(T):
    ans = []
    for i in range(N):
        ans.append(g2(T.fold(0, i + 1))[0])
    print("test", *ans)
    
    
def check(m):
    T = LazySegmentTree(N+1,
                        0,
                        0,
                        op,
                        add,
                        reflect_f)
    
    T.build([1 for _ in range(N + 1)])
    for x, w in zip(X, W):
        d = w // m
        r = min(x + d, N - 1)
        v = w - (r - x) * m
        T = f(x, w, r, v, -m, T)
        # printT(T)
        w -= m
        x -= 1
        if w <= 0 or x < 0:
            continue
        l = max(0, x - d)
        v = w - (x - l) * m
        T = f(l, v, x, w, m, T)
        # printT(T)
        
    for i in range(N):
        v = g2(T.fold(0, i + 1))[0]
        if v >= A[i]:
            return False
        
    return True 
        
sw = sum(W)
flag = 1
for i in range(N):
    if A[i] <= sw:
        flag = 0
if flag:
    print(0)
    exit()

yes = 10 ** 9 + 5
no = 0
if not check(yes):
    print(-1)
    exit()
    
while yes - no != 1:
    mid = (yes + no)//2
    if check(mid):
        yes = mid
    else:
        no = mid
    
print(yes)