Console readline query

Question

Console.WriteLine("Program to calculate weight of man on moon by given weight on earth");

Console.WriteLine("Enter mass on earth");

Console.ReadLine();

int m1 = int.Parse(Console.ReadLine());

Console.WriteLine("Enter gravity on earth");

int g1 = int.Parse(Console.ReadLine());

int weightonearth = m1 * g1;

Console.WriteLine("Weight on earth is " + " " + weightonearth);

Console.ReadLine();

Console.WriteLine("Now lets calculate weight on moon based on weight on earth");

double weightonmoon = 0.17 * weightonearth;

Console.WriteLine("Weight on moon is " + " " + weightonmoon);

The above program is throwing exception error . Why?

but, if i give like below its not throwing error.

Console.WriteLine("Program to calculate weight of man on moon by given weight on earth");

Console.WriteLine("Enter mass on earth");

string a= Console.ReadLine();

int m1 = int.Parse(a);

Console.WriteLine("Enter gravity on earth");

int g1 = int.Parse(Console.ReadLine());

int weightonearth = m1 * g1;

Console.WriteLine("Weight on earth is " + " " + weightonearth);

Console.ReadLine();

Console.WriteLine("Now lets calculate weight on moon based on weight on earth");

double weightonmoon = 0.17 * weightonearth;

Console.WriteLine("Weight on moon is " + " " + weightonmoon);

 

In this way if it is correct then int g1 = int.Parse(Console.ReadLine()); should also throw me exception error right? But its not throwing error.

One more question is : does console.readline always assume the datatype as string?