## A - Find Multiple 解説 by en_translator

If you are new to learning programming and do not know where to start, please try Problem A “Welcome to AtCoder” from practice contest. There you can find a sample code for each language.

Let $$X$$ be the quotient of $$B$$ divided by $$C$$, with the remainder rounded down, that is, $$X=\lfloor \frac{B}{C} \rfloor$$.

Let $$Y=X \times C$$.
Then $$Y$$ is the minimum multiple of $$C$$ not exceeding $$B$$.

Therefore, if $$Y \geq A$$, then $$Y$$ is an answer.
If $$Y \lt A$$, then the answer is $$-1$$.

A sample code in C++ follows.

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;

signed main(){
ll a,b,c;
cin>>a>>b>>c;
ll Y = b / c * c; // Y is a multiplt of C not exceeding B
if(a <= Y){
cout<<Y<<endl;
}else{
cout<<-1<<endl;
}
return 0;
}