#include <bits/stdc++.h>

typedef long long ll;
const int maxn = 2e5 + 3;

int n, m, x[maxn];
ll a[maxn], w[maxn];

ll ceildiv(ll a, ll b) {
    return a % b ? a / b + 1 : a / b;
}

ll pt[maxn][2];
bool check(ll c) {
    if(c <= 0) {
        return std::accumulate(w, w + m, 0ll) <= *std::min_element(a + 1, a + n + 1);
    }
    for(int i = 1; i <= n + 1; ++i)
        pt[i][0] = pt[i][1] = 0;
    for(int j = 0; j < m; ++j) {
        ll u = w[j] / c + x[j];
        ll v = x[j] - w[j] / c;
        pt[std::max(1ll, v)][0] += c;
        pt[std::max(1ll, v)][1] += w[j] - c * x[j];
        pt[x[j]][0] += -c - c;
        pt[x[j]][1] += w[j] + c * x[j] - (w[j] - c * x[j]);
        pt[std::min(n + 1ll, u + 1)][0] += c;
        pt[std::min(n + 1ll, u + 1)][1] += -(w[j] + c * x[j]);
    }
    ll u = 0, v = 0;
    for(int i = 1; i <= n; ++i) {
        u += pt[i][0];
        v += pt[i][1];
        if(u * (ll)i + v >= a[i])
            return false;
    }
    return true;
}

int main() {
    scanf("%d%d", &n, &m);
    for(int i = 1; i <= n; ++i)
        scanf("%lld", a + i);
    for(int i = 0; i < m; ++i)
        scanf("%d%lld", x + i, w + i);
    ll l = -1, r = 1e10;
    while(r - l > 1) {
        ll mid = (l + r) / 2;
        if(check(mid))
            r = mid;
        else
            l = mid;
    }
    printf("%lld\n", r < 1e10 ? r : -1);
    return 0;
}