package contest200306;
import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.InputMismatchException;

public class F {
	InputStream is;
	PrintWriter out;
	String INPUT = "";
	
	void solve()
	{
		int n = ni(), m = ni();
		int[] a = na(n);
		int[] xs = new int[m];
		int[] ws = new int[m];
		for(int i = 0;i < m;i++) {
			xs[i] = ni()-1;
			ws[i] = ni();
		}
		
		long[] oo = new long[n];
		for(int i = 0;i < m;i++) {
			oo[xs[i]] += ws[i];
		}
		boolean ook = true;
		for(int i = 0;i < n;i++) {
			if(oo[i] >= a[i]) {
				ook = false;
			}
		}
		if(!ook) {
			out.println(-1);
			return;
		}
		
		long alsum = 0;
		for(int i = 0;i < m;i++) {
			alsum += ws[i];
		}
		ook = true;
		for(int i = 0;i < n;i++) {
			if(alsum >= a[i]) {
				ook = false;
			}
		}
		if(ook) {
			out.println(0);
			return;
		}
		
//		ok(1, a, xs, ws);
		
		long low = 0, high = (long)1e14+10;
		while(high - low > 1) {
			long h = high+low>>1;
			if(ok(h, a, xs, ws)) {
				high = h;
			}else{
				low = h;
			}
		}
		out.println(high);
	}
	
	boolean ok(long h, int[] a, int[] xs, int[] ws)
	{
		int n = a.length;
		int m = xs.length;
		
		long[] imos1 = new long[n+5];
		long[] imos = new long[n+5];
		for(int i = 0;i < m;i++) {
			int x = xs[i];
			int w = ws[i];
			
			long d = w/h;
			long rem = w%h;
//			tr(x, d, w);
			if(x-d >= 0) {
				plus(imos, x-d, rem);
				plus(imos1, x-d+1, h);
				plus(imos1, x+1, -h);
			}else if(x-d+1 >= 0) {
				plus(imos, 0, rem);
				plus(imos1, x-d+1, h);
				plus(imos1, x+1, -h);
			}else {
				plus(imos, 0, rem+h*-(x-d+1));
				plus(imos1, 0, h);
				plus(imos1, x+1, -h);
			}
			plus(imos, x+d+1, -rem);
			plus(imos1, x+1, -h);
			plus(imos1, x+d+1, h);
//			tr(imos);
//			tr(imos1);
		}
		
		for(int i = 0;i < imos1.length-1;i++) {
			imos1[i+1] += imos1[i];
		}
//		tr(h, imos1);
		for(int i = 0;i < imos1.length;i++) {
			imos[i] += imos1[i];
		}
		for(int i = 0;i < imos.length-1;i++) {
			imos[i+1] += imos[i];
		}
//		tr(imos);
		for(int i = 0;i < a.length;i++) {
			if(imos[i] >= a[i])return false;
		}
		return true;
	}
	
	void plus(long[] a, long i, long x)
	{
		if(0 <= i && i < a.length)a[(int)i] += x;
	}
	
	void run() throws Exception
	{
		is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes());
		out = new PrintWriter(System.out);
		
		long s = System.currentTimeMillis();
		solve();
		out.flush();
		if(!INPUT.isEmpty())tr(System.currentTimeMillis()-s+"ms");
//		Thread t = new Thread(null, null, "~", Runtime.getRuntime().maxMemory()){
//			@Override
//			public void run() {
//				long s = System.currentTimeMillis();
//				solve();
//				out.flush();
//				if(!INPUT.isEmpty())tr(System.currentTimeMillis()-s+"ms");
//			}
//		};
//		t.start();
//		t.join();
	}
	
	public static void main(String[] args) throws Exception { new F().run(); }
	
	private byte[] inbuf = new byte[1024];
	public int lenbuf = 0, ptrbuf = 0;
	
	private int readByte()
	{
		if(lenbuf == -1)throw new InputMismatchException();
		if(ptrbuf >= lenbuf){
			ptrbuf = 0;
			try { lenbuf = is.read(inbuf); } catch (IOException e) { throw new InputMismatchException(); }
			if(lenbuf <= 0)return -1;
		}
		return inbuf[ptrbuf++];
	}
	
	private boolean isSpaceChar(int c) { return !(c >= 33 && c <= 126); }
	private int skip() { int b; while((b = readByte()) != -1 && isSpaceChar(b)); return b; }
	
	private double nd() { return Double.parseDouble(ns()); }
	private char nc() { return (char)skip(); }
	
	private String ns()
	{
		int b = skip();
		StringBuilder sb = new StringBuilder();
		while(!(isSpaceChar(b))){ // when nextLine, (isSpaceChar(b) && b != ' ')
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}
	
	private char[] ns(int n)
	{
		char[] buf = new char[n];
		int b = skip(), p = 0;
		while(p < n && !(isSpaceChar(b))){
			buf[p++] = (char)b;
			b = readByte();
		}
		return n == p ? buf : Arrays.copyOf(buf, p);
	}
	
	private int[] na(int n)
	{
		int[] a = new int[n];
		for(int i = 0;i < n;i++)a[i] = ni();
		return a;
	}
	
	private long[] nal(int n)
	{
		long[] a = new long[n];
		for(int i = 0;i < n;i++)a[i] = nl();
		return a;
	}
	
	private char[][] nm(int n, int m) {
		char[][] map = new char[n][];
		for(int i = 0;i < n;i++)map[i] = ns(m);
		return map;
	}
	
	private int[][] nmi(int n, int m) {
		int[][] map = new int[n][];
		for(int i = 0;i < n;i++)map[i] = na(m);
		return map;
	}
	
	private int ni() { return (int)nl(); }
	
	private long nl()
	{
		long num = 0;
		int b;
		boolean minus = false;
		while((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'));
		if(b == '-'){
			minus = true;
			b = readByte();
		}
		
		while(true){
			if(b >= '0' && b <= '9'){
				num = num * 10 + (b - '0');
			}else{
				return minus ? -num : num;
			}
			b = readByte();
		}
	}
	
	private static void tr(Object... o) { System.out.println(Arrays.deepToString(o)); }
}