C++error 2064：项不会接受一个参数的函数

#include
#include
using namespace std;
double f(double x1,double y1,double x2,double y2)
{double L;
L=sqrt((x1-y2)(x1-y2)+(x1-y2)(x1-y2));
return L;}
int main()
{double a1,a2,b1,b2,distance;
cout<<"please enter x1's Abscissa and ordinate:";
cin>>a1>>b1;
cout<<"please enter x2's Abscissa and ordinate:";
cin>>a2>>b2;
distance=f(a1,b1,a2,b2);
cout<<"the distandce is"<<distance<<endl;
return 0;

可能你输入的值(x1-y2)(x1-y2)+(x1-y2)(x1-y2)的结果为负数