Getting more Calories

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As all of us are hardgainers, we tend to hear "eat more" or something along those lines to gain weight and since i started counting calories I've only been eating like 1,000-1,600 calories a day, sometimes 2,000 on a good day. But i've gained only roughly 10 pounds in the last 4 months. My question is what foods are extremely high in calories and good for gaining weight?