#include<bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef pair<int,int> P;

#define p_ary(ary,a,b) do { cout << "["; for (int count = (a);count < (b);++count) cout << ary[count] << ((b)-1 == count ? "" : ", "); cout << "]\n"; } while(0)
#define p_map(map,it) do {cout << "{";for (auto (it) = map.begin();;++(it)) {if ((it) == map.end()) {cout << "}\n";break;}else cout << "" << (it)->first << "=>" << (it)->second << ", ";}}while(0)

template<typename T1,typename T2>ostream& operator<<(ostream& os,const pair<T1,T2>& a) {os << "(" << a.first << ", " << a.second << ")";return os;}

struct LazySegmentTree {
private:
    int n;
    vector<ll> node,lazy;
public:
    LazySegmentTree(vector<ll> v) {
        int sz = v.size();
        n = 1;
        while (n < sz) n *= 2;
        node.resize(2*n-1);
        lazy.resize(2*n-1,0);
        for (int i = 0;i < sz;++i) node[i+n-1] = v[i];
        for (int i = n-2;i >= 0;--i) node[i] = node[i*2+1]+node[i*2+2];
    }

    void eval(int k,int l,int r) {
        if (lazy[k] != 0) {
            node[k] += lazy[k];
            if (r-l > 1) {
                lazy[2*k+1] += lazy[k]/2;
                lazy[2*k+2] += lazy[k]/2;
            }
            lazy[k] = 0;
        }
    }

    void add(int a,int b,ll x,int k = 0,int l = 0,int r = -1) {
        if (r < 0) r = n;
        eval(k,l,r);
        if (b <= l || r <= a) return;
        if (a <= l && r <= b) {
            lazy[k] += (r-l)*x;
            eval(k,l,r);
        } else {
            add(a,b,x,2*k+1,l,(l+r)/2);
            add(a,b,x,2*k+2,(l+r)/2,r);
            node[k] = node[2*k+1]+node[2*k+2];
        }
    }

    ll getsum(int a,int b,int k = 0,int l = 0,int r = -1) {
        if (r < 0) r = n;
        if (b <= l || r <= a) return 0;
        eval(k,l,r);
        if (a <= l && r <= b) return node[k];
        ll vl = getsum(a,b,k*2+1,l,(l+r)/2);
        ll vr = getsum(a,b,2*k+2,(l+r)/2,r);
        return vl+vr;
    }
};

int main() {
    int n,m;
    cin >> n >> m;
    vector<ll> a(n),w(m),sum(n,0);
    vector<int> x(m);
    for (int i = 0;i < n;++i) scanf("%lld",&a[i]);
    for (int i = 0;i < m;++i) scanf("%d%lld",&x[i],&w[i]),x[i]--;
    for (int i = 0;i < m;++i) sum[x[i]] += w[i];
    for (int i = 0;i < n;++i) if (a[i] < sum[i]) {
        cout << -1 << endl;
        return 0;
    }
    ll s = 0;
    for (int i = 0;i < m;++i) s += w[i];
    bool f = true;
    for (int i = 0;i < n;++i) if (a[i] < s) {f = false;break;}
    if (f) {
        cout << 0 << endl;
        return 0;
    }
    ll mn = 0,mx = 1e9;
    while (mx-mn > 1) {
        int mid = (mn+mx)/2;
        LazySegmentTree seg1(vector<ll>(n,0)),seg2(vector<ll>(n,0)),seg3(vector<ll>(n,0));
        for (int i = 0;i < m;++i) {
            int c = w[i]/mid;
            seg1.add(x[i]-c,x[i]+c+1,w[i]);
            seg2.add(x[i]+1,x[i]+1+c,mid);
            seg2.add(x[i]+1+c,x[i]+2+c,-(ll)c*mid);
            seg3.add(x[i]-c,x[i],mid);
            seg3.add(x[i]-c-1,x[i]-c,-(ll)c*mid);
        }
        bool ok = true;
        for (int i = 0;i < n;++i) if (seg1.getsum(i,i+1)-seg2.getsum(0,i+1)-seg3.getsum(i,n) > a[i]) {
            ok = false;
            break;
        }
        (ok ? mx : mn) = mid;
    }
    cout << mx << endl;
}