AC但不理解

#include<bits/stdc++.h>
using namespace std;
#define ll long long
ll A,B,C,a,b,m,n,x,y,L;
ll exgcd(ll a,ll b,ll &x,ll &y){
	if(b==0){
		x=1,y=0;
		return a;
	}
	ll px,py;
	ll d=exgcd(b,a%b,px,py);
	x=py;
	y=px-a/b*py;
	return d;
}
int main(){
	while(cin >> a >> b >> m >> n >> L){
		A=n-m,B=L,C=a-b;
		ll d=exgcd(A,B,x,y);
		if(C%d!=0) cout << "Impossible\n";
		else{
			x*=(C/d);
			if(x<0) x+=(-x/B+1)*B;
			cout << x%(B/d) << "\n";
		}
	}
	return 0;
}