#include <iostream>
#include <cmath>
using namespace std;
 
int x1, x2, y1, y2;
 
int main(){
    cin >> x1 >> y1 >> x2 >> y2;
    int xd = x2 - x1;
    int yd = y2 - y1;
    double dist = sqrt(xd*xd + yd*yd);
    cout << dist;
}

Here are some things it's not:
- "#include <cmath>" plus "using namespace std;" is essentially equivalent to "#include <math.h>". Yes, the compiler manages to find "sqrt".
- Number coercion (the int-to-double lurking in the computational line) works fine.
- Using <iostream> and <cmath> don't conflict or anything.
- No, it's not about ending with a "return 0;"

I don't know why I'm doing this; it feels like such a natural possible mistake that maybe the C++ programmers who will read this already know.
Ready for the answer?