#include<stdio.h>
int main()
{
    long long a,b,n,x,sum=0,max;
    scanf("%lld %lld %lld",&a,&b,&n);
    x=n-1;
    sum=(a*x/b)-5*(x/b);
    max=sum;
    x=n;
    sum=(a*x/b)-5*(x/b);
    if(sum>max)
    {
        printf("%lld",sum);
    }
    else
    {
        printf("%lld",max);
    }
    return 0;
}
