MonoGame kerning problem

using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;

namespace GameName1
{
    /// <summary>
    /// This is the main type for your game
    /// </summary>
    public class Game1 : Game
    {
        GraphicsDeviceManager graphics;
        SpriteBatch spriteBatch;
        SpriteFont tempFont;

        public Game1()
            : base()
        {
            graphics = new GraphicsDeviceManager(this);
            Content.RootDirectory = "Content";
        }

        protected override void LoadContent()
        {
            spriteBatch = new SpriteBatch(GraphicsDevice);

            tempFont = Content.Load<SpriteFont>("SmallText");
        }

        protected override void UnloadContent()
        {
        }

        protected override void Update(GameTime gameTime)
        {
            if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed || Keyboard.GetState().IsKeyDown(Keys.Escape))
                Exit();

            base.Update(gameTime);
        }

        protected override void Draw(GameTime gameTime)
        {
            // Sample draw
            graphics.GraphicsDevice.Clear(new Color(204, 204, 204));

            spriteBatch.Begin();
            spriteBatch.DrawString(tempFont, "Ahoj, jak se mbm", new Vector2(20, 20), Color.Red);
            spriteBatch.DrawString(tempFont, "Ahoj, jak se mb", new Vector2(20, 20), Color.Blue);
            float size = tempFont.MeasureString("Ahoj, jak se mbm").X - tempFont.MeasureString("m").X;
            spriteBatch.DrawString(tempFont, "m", new Vector2(20 + size, 20), Color.Yellow);
            spriteBatch.End();

            base.Draw(gameTime);
        }
    }
}