NOIP2012 同余方程

#include <cstdio>
#include <algorithm>
#include <cstring>
using namespace std;
void exgcd(long long int a,long long int b,long long int &x,long long int &y)
{
if(b==0)
{
x=1;
y=0;
}
else {
long long int tx,ty;
exgcd(b,a%b,tx,ty);
x=ty;
y=tx-(a/b)*ty;
}
return;
}
int main()
{
long long int a,b,x,y;
scanf("%lld %lld",&a,&b);
exgcd(a,b,x,y);
x%=b;
if(x<0) x+=b;
printf("%lld",x);
return 0;
}