using System;

using System.Collections.Generic;

public class Program {

  private static double Interpolate(List<(double x, double y)> list, double x) {

    if (list.Count == 0)

      return double.NaN;

​

    if (list.Count == 1)

      return list[0].y;

​

    var index = list.BinarySearch((x, 0),

      Comparer<(double x, double y)>.Create((left, right) => left.x.CompareTo(right.x)));

​

    if (index >= 0)

      return list[index].y;

​

    int leftIndex = ~index - 1;

​

    if (leftIndex < 0)

      leftIndex = 0;

​

    int rightIndex = leftIndex + 1;

​

    if (rightIndex == list.Count) {

      leftIndex -= 1;

      rightIndex -= 1;

    }

​

    double xa = list[leftIndex].x;

    double ya = list[leftIndex].y;

​

    double xb = list[rightIndex].x;

    double yb = list[rightIndex].y;

​

    return ya + (x - xa) / (xb - xa) * (yb - ya);

  } 

  public static void Main() {

    var data = new List<(double x, double y)> {

      (1, 123),

      (2, 234),

      (3, 345),

    };

    var result = Interpolate(data, 1.5);

    Console.WriteLine(result);

  }

}