CF466A Cheap Travel

Ann has recently started commuting by subway. We know that a one ride subway ticket costs $ a $ rubles. Besides, Ann found out that she can buy a special ticket for $ m $ rides (she can buy it several times). It costs $ b $ rubles. Ann did the math; she will need to use subway $ n $ times. Help Ann, tell her what is the minimum sum of money she will have to spend to make $ n $ rides?

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In the first sample one of the optimal solutions is: each time buy a one ride ticket. There are other optimal solutions. For example, buy three $ m $ ride tickets.