Fibo sum of operations

public BigInteger iterativeF(int n) {
        BigInteger a = BigInteger.ONE;
        BigInteger b = BigInteger.ZERO;

        for (int i = 0; i < n; i++) {
            b = b.add(a);
            a = b.subtract(a);
        }
        return b;
    }

    public BigInteger sumOfOperations(BigInteger lastFiboNumber) {
        return BigInteger.ONE.add(lastFiboNumber).divide(BigInteger.TWO).multiply(BigInteger.valueOf(5));

    }