var input = Console.ReadLine().Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries).Select(int.Parse).ToArray();

            var rows = input[0];
            var cols = input[1];
            var limeTreeRow = input[2];
            var limeTreeRCol = input[3];

            var currentValue = 0;

            var limeField = new int[rows,cols];

            for (int row = 0; row < limeField.GetLength(0); row++)
            {
                for (int col = 0; col < limeField.GetLength(1); col++)
                {
                    limeField[row, col] = currentValue;
                    currentValue++;
                }
            }

            Console.WriteLine(limeField[limeTreeRow - 1, limeTreeRCol - 1]);