giving n positive integers a1,a2,...,an. gcd(a1,a2,...,an)=1. define f(x1,x2,...,xn)=a1*x1+a2*x2+a3*x3+...+an*xn(xi>=0). what is the maximum number b that can not be expressed by f(x1,x2,...,xn)? x1,x2,...,xn are all non-negative integers.

I already know the answer for n=2, it's equal to a1*a2-a1-a2. but when n>2,I can't get it. can anyone give me some hints?