This led to the concept of the cost-exchange ratio, the amount of money one had to spend on additional defenses to counter a dollar of new offensive capability. Early estimates were around 20, meaning every dollar the Soviets spent on new ICBMs would require the US to spend $20 to counter it. This implied the Soviets could afford to overwhelm the US's ability to build more interceptors. With MIRV, the cost-exchange ratio was so one-sided there was no effective defense that could not be overwhelmed for little cost, as mentioned in a famous 1968 article by Bethe and Garwin. This is precisely what the US did when the Soviets installed their A-35 anti-ballistic missile system around Moscow; by adding MIRV to the Minuteman missile fleet, they could overwhelm the A-35 without adding a single new missile.