function annualInterest() public constant returns(uint interest) {
        uint _now = now;
        interest = maxMintProofOfStake;
        if((_now.sub(stakeStartTime)).div(1 years) == 0) {
            interest = (1650 * maxMintProofOfStake).div(100);
        } else if((_now.sub(stakeStartTime)).div(1 years) == 1) {
            interest = (770 * maxMintProofOfStake).div(100);
        } else if((_now.sub(stakeStartTime)).div(1 years) == 2){
            interest = (435 * maxMintProofOfStake).div(100);
        }
    }