Pi computing

#include <stdio.h>
#include <stdlib.h>

unsigned long long pow(int x, int y)
{
    unsigned long long res;
    res = x;
    if(y<1)
    {
        return 1;
    }
    else
    {
        for(int i=1;i<y;i++)
        {
            res=res*x;
        }
        return res;
    }
}


int main(void)
{
    unsigned long long  main_mass=10;
    int ratio, secondary_mass=1;
    double main_vel=1, secondary_vel=0, vcm;
    unsigned long long  collisions=0;
    do
    {
        printf("input the weight of the main mass(positive) A, so that weight=100^A, the secondary mass is 1kg");
        scanf("%d",&ratio);
        main_mass=pow(100,ratio);
    }
    while(ratio<0);


    do
    {
        //cubes collide
        vcm = (main_mass*main_vel+secondary_mass*secondary_vel)/(main_mass+secondary_mass);
//      printf("2*%f-%f=",vcm,main_vel);
        main_vel = (2*vcm-main_vel);
//      printf("%f\n",main_vel);
        secondary_vel = 2*vcm-secondary_vel;
        collisions++;
        if(secondary_vel>0){
        //wall and cube collide
        secondary_vel = secondary_vel*(-1);
        collisions ++;}

    }
    while( secondary_vel < main_vel ||( secondary_vel>0 && main_vel > 0));
    printf("%llu collisions\n",collisions);

    return 0;


}