hdu 1788 Chinese remainder theorem again

#include<iostream>
#include<cstdio>
using namespace std;

__int64 gcd(__int64 a,__int64 b)
{
if(b==0) return a;
return gcd(b,a%b);
}

int main()
{
int n,a,i,c,num;
__int64 dd;
while(scanf("%d%d",&n,&a)&&n||a)
{
scanf("%I64dd",&dd);
for(i=1;i<n;i++)
{
scanf("%d",&num);
c=gcd(dd,num);//
dd=dd/c*num;//
}
printf("%I64d\n",dd-a);
}
return 0;
}