#include <iostream>
using namespace std;
typedef long long ll;
ll N,M;
ll A[100010] = {},Lsum[100010] = {},Rsum[100010] = {};
ll B[100010] = {};
ll Limos[100010] = {},Rimos[100010] = {};
ll R[100010],S[100010];

bool ok(ll m){
    for(int i=0;i<=N+1;i++){
        Lsum[i] = 0; Rsum[i] = 0;
        Limos[i] = 0; Rimos[i] = 0;
    }
    for(int i=1;i<=M;i++){
        Lsum[R[i]] += S[i]; Rsum[R[i]] += S[i];
        if(m!=0){Limos[R[i]+1]++; Limos[min(R[i]+S[i]/m+1,N+1)]--; Lsum[min(R[i]+S[i]/m+1,N+1)] -= S[i]%m;}
        if(m!=0){Rimos[R[i]-1]++; Rimos[max(R[i]-S[i]/m-1,0LL)]--; Rsum[max(R[i]-S[i]/m-1,0LL)] -= S[i]%m;}
    }
    for(int i=1;i<=N;i++){
        Limos[i] += Limos[i-1];
        Lsum[i] += Lsum[i-1]-m*Limos[i];
    }
    for(int i=N;i>=1;i--){
        Rimos[i] += Rimos[i+1];
        Rsum[i] += Rsum[i+1]-m*Rimos[i];
    }
/*    cerr << "m:" << m << endl;
    cerr << "fuan:";
    for(int i=1;i<=N;i++) cerr << Rsum[i]+Lsum[i]-B[i] << " ";
    cerr << endl;
    cerr << "Limos:";
    for(int i=1;i<=N;i++) cerr << Limos[i] << " ";
    cerr << endl;
    cerr << "Rimos:";
    for(int i=1;i<=N;i++) cerr << Rimos[i] << " ";
    cerr << endl;
*/    for(int i=1;i<=N;i++){
        if(Rsum[i]+Lsum[i]-B[i]>=A[i]) return false;
    }
    return true;
}

int main(){
    cin >> N >> M;
    for(int i=1;i<=N;i++) cin >> A[i];
    for(int i=1;i<=M;i++){
        cin >> R[i] >> S[i];
        B[R[i]] += S[i];
    }
    for(int i=1;i<=N;i++){
        if(B[i]>=A[i]){
            cout << -1 << endl;
            return 0;
        }
    }
    ll l = -1,r = 1e6;
    while(l+1<r){
        ll m = (l+r)/2;
        if(ok(m)) r = m;
        else l = m;
    }
    cout << r << endl;
}