#include <bits/stdc++.h>
int n, m;
int d[250000];
int h[250000];
int c[250000];
int r[250000];
int diferencias[160000];
int a[401];
bool sirve(int v, int idCamion) {
  int litros = v;
  int cantRecargas = r[idCamion];
  for (int i = d[idCamion] + 1; i <= h[idCamion]; i++) {
    if (v < a[i] - a[i - 1]) {
      return false;
    }
    if (litros < a[i] - a[i - 1]) {
      if (cantRecargas) {
        litros = v;
        cantRecargas--;
      } else {
        return false;
      }
    }
    litros -= a[i] - a[i - 1];
  }
  return true;
}
int cmpfunc(const void* a, const void* b) { return (*(int*)a - *(int*)b); }
int main() {
  scanf("%d %d", &n, &m);
  for (int i = 0; i < n; i++) scanf("%d", &a[i + 1]);
  for (int i = 0; i < m; i++) scanf("%d %d %d %d", &d[i], &h[i], &c[i], &r[i]);
  int tam2 = 0;
  for (int i = 1; i <= n; i++)
    for (int j = i; j <= n; j++) diferencias[tam2++] = a[j] - a[i];
  qsort(diferencias, tam2, sizeof(int), cmpfunc);
  int tam = 1;
  for (int i = 1; i < tam2; i++)
    if (diferencias[i] != diferencias[tam - 1])
      diferencias[tam++] = diferencias[i];
  long long minV = 0;
  int ini, fin, mid;
  for (int i = 0; i < m; i++) {
    ini = 0;
    fin = tam - 1;
    while (ini + 1 < fin) {
      mid = (ini + fin) / 2;
      if (sirve(diferencias[mid], i))
        fin = mid;
      else
        ini = mid;
    }
    if (sirve(diferencias[ini], i))
      minV = (minV > ((long long)diferencias[ini]) * ((long long)c[i])
                  ? minV
                  : ((long long)diferencias[ini]) * ((long long)c[i]));
    else
      minV = (minV > ((long long)diferencias[fin]) * ((long long)c[i])
                  ? minV
                  : ((long long)diferencias[fin]) * ((long long)c[i]));
  }
  printf("%I64d\n", minV);
  return 0;
}
