The monsoon umbrellas algorithm finds the minimum number of umbrellas that can cover a given number of people and no more.

GetMinimumUmbrellas.cs

using System;
using System.Collections.Generic;

namespace Monsoon_Umbrellas_Algorithm
{
    class Program
    {

        public static int GetMinimumUmbrellas(int[] sizes, int requirement)
        {
            int result = 0;
            int arrayAddition = 0;
            int i = 0;
            int mid = sizes.Length / 2;

            Array.Sort(sizes);

            if (sizes.Length > 2)
            {

                while (mid > i)
                {
                    arrayAddition += sizes[mid];

                    if (sizes[mid] == requirement)
                    {
                        result = 1;
                    }
                    else if (arrayAddition == requirement)
                    {
                        result = i + 1;
                    }
                    else
                    {
                        result = -1;
                    }

                    mid--;
                    i++;

                }

            }
            else
            {
                for (int j = 0; j < sizes.Length; j++)
                {
                    arrayAddition += sizes[j];

                    if (sizes[j] == requirement)
                    {
                        result = 1;
                    }
                    else if (arrayAddition == requirement)
                    {
                        result = j + 1;
                    }
                    else
                    {
                        result = -1;
                    }
                }
            }

            return result;

        }

        static void Main(string[] args)
        {
            //int[] sizes = new int[2] { 3, 5 };
            //int requirement = 5;

            int[] sizes = new int[7] { 1, 2, 3, 4, 5, 6, 9 };
            int requirement = 7;

            int result = GetMinimumUmbrellas(sizes, requirement);

            Console.WriteLine(result.ToString());
        }

    }
}