666,WA了5个点

#include<bits/stdc++.h>
using namespace std;
double a1,b1,a2,b2;
void f(double x1,double x2,double y1,double y2){
	cout<<fixed<<setprecision(3)<<sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));    
}
int main(){
	cin>>a1>>b1>>a2>>b2;
	f(a1,b1,a2,b2);
   return 0;
}

6