to find the square root of a number without using predfined function

#include<iostream>
#include<conio.h>
using namespace std;

int main()
{ float g,r,c=0,n,pg;  //g guess pg previous guess
cout<<"enter the number to find squareroot   ";
cin>>n;
        g=n/2; pg=n;
       while(g!=pg)
       {r=n/g;
       pg=g;
       g=(g+r)/2;
       }
cout<<"square root \t"<<g<<endl;
getch();