this is geometrik progress.
read(s, min, p);
q := 1-p/100;
then..
B[n]=b[1]*q^(n-1)  that's why
min < s*q^(n-1)
q^(n-1)>min/s   q < 1  then  ln(q)<0
n-1 < ln(min/s)/ln(q);
n < ln(min/s)/ln(q)+1