import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }




void main() {
  try {
    for (; ; ) {
      const N = readInt();
      const M = readInt();
      auto A = new long[N];
      foreach (i; 0 .. N) {
        A[i] = readLong();
      }
      auto X = new int[M];
      auto W = new long[M];
      foreach (j; 0 .. M) {
        X[j] = readInt() - 1;
        W[j] = readLong();
      }
      
      bool check(long c) {
        auto sum0 = new long[N + 1];
        auto sum1 = new long[N + 1];
        foreach (j; 0 .. M) {
          if (c == 0) {
            sum0[0] += W[j];
          } else {
            const lb = cast(int)(max(X[j] - W[j] / c, 0));
            const ub = cast(int)(min(X[j] + W[j] / c + 1, N));
            if (lb < ub) {
              sum0[lb] += W[j] - c * X[j];
              sum0[X[j]] -= W[j] - c * X[j];
              sum1[lb] += c;
              sum1[X[j]] -= c;
              sum0[X[j]] += W[j] + c * X[j];
              sum0[ub] -= W[j] + c * X[j];
              sum1[X[j]] += -c;
              sum1[ub] -= -c;
            }
          }
        }
        foreach (i; 0 .. N) {
          sum0[i + 1] += sum0[i];
          sum1[i + 1] += sum1[i];
        }
        foreach (i; 0 .. N) {
          if (sum0[i] + sum1[i] * i >= A[i]) {
            return false;
          }
        }
        return true;
      }
      
      long lo = -1, hi = W.maxElement;
      if (!check(hi)) {
        writeln(-1);
      } else {
        for (; lo + 1 < hi; ) {
          const mid = (lo + hi) / 2;
          (check(mid) ? hi : lo) = mid;
        }
        writeln(hi);
      }
    }
  } catch (EOFException e) {
  }
}