Because I think it is not really profitable to buy miners for bitcoins, I invested mining other coins.

A friend of mine did something much more profitable. He bought one BTC and started trading.
He bought for 600 euro and then sold for 650 euro. Then bought at 630 euro and sold at 645 euro.
Redo this trick a month long. He made 4,3 BTC out of 1 BTC. He was able to make about 80 euro every day.
If he continues like this, next month he could have 16 BTC.