#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;}

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;
        vector<ll> sum1(n,0),sum2(n,0),sum3(n,0);
        for (int i = 0;i < m;++i) {
            int c = w[i]/mid;
            sum1[max(0,x[i]-c)] += w[i];
            if (x[i]+c+1 < n) sum1[x[i]+c+1] -= w[i];
            if (x[i]+1 < n) sum2[x[i]+1] += mid;
            if (x[i]+1+c < n) sum2[x[i]+c+1] -= mid;
            if (x[i]-1 >= 0) sum3[x[i]-1] += mid;
            if (x[i]-1-c >= 0) sum3[x[i]-1-c] -= mid;
        }
        for (int i = 1;i < n;++i) sum1[i] += sum1[i-1];
        for (int i = 1;i < n;++i) sum2[i] += sum2[i-1];
        for (int i = n-1;i > 0;--i) sum3[i-1] += sum3[i];
        for (int i = 0;i < m;++i) {
            int c = w[i]/mid;
            if (x[i]+1+c < n) sum2[x[i]+c+1] -= (ll)c*mid;
            if (x[i]-c-1 >= 0) sum3[x[i]-c-1] -= (ll)c*mid;
        }
        for (int i = 1;i < n;++i) sum2[i] += sum2[i-1];
        for (int i = n-1;i > 0;--i) sum3[i-1] += sum3[i];
        bool ok = true;
        for (int i = 0;i < n;++i) if (sum1[i]-sum2[i]-sum3[i] > a[i]) {
            ok = false;
            break;
        }
        (ok ? mx : mn) = mid;
    }
    cout << mx << endl;
}