public long calculate(long number){
			if(number < 0){
				throw new IllegalArgumentException(number + " is illegal");
			}
			long prev1 = 0L;
			long prev2 = 1L;
			for (int i = 0; i < number; i++) {
				long value = prev2 + prev1;
				prev2 = prev1;
				prev1 = value;
			}
			return prev1;
		}