// rope
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#define pi 3.14159265358979
//
int N;
double Ans,R;
double *X;
double *Y;
double *C;
double *Z;
double *D;
//
void Init(void);
void Process(void);
//
void main(void)
{
    Init();
    Process();
}
//
void Init(void)
{
    int i;
    scanf("%d%lf",&N,&R);
    X=(double *)malloc((N+2)*sizeof(double));
    Y=(double *)malloc((N+2)*sizeof(double));
    D=(double *)malloc((N+2)*sizeof(double));
    C=(double *)malloc((N+2)*sizeof(double));
    Z=(double *)malloc((N+2)*sizeof(double));
    for (i=1;i<=N;i++)
    {
        scanf("%lf%lf",&X[i],&Y[i]);
    }
    X[0]=X[N];
    Y[0]=Y[N];
    X[N+1]=X[1];
    Y[N+1]=Y[1];
}
//
void Process(void)
{
    int i,j;
    double alpha1,alpha2;
    //find Diff
    for (i=1;i<=N;i++)
    {
        D[i]=sqrt(pow((X[i]-X[i+1]),2)+pow((Y
[i]-Y[i+1]),2));
    }
    //find Zeta
    for (i=1;i<=N;i++)
    {
        alpha1=atan2(abs(Y[i+1]-Y[i]),abs(X
[i+1]-X[i]));
        alpha2=atan2(abs(Y[i]-Y[i-1]),abs(X[i]-
X[i-1]));
        Z[i]=pi-(alpha1+alpha2);
    }
    //find Curve
    for (i=1;i<=N;i++)
    {
        C[i]=Z[i]*R;
    }
    //find sum Answer
    Ans=0;
    for (i=1;i<=N;i++)
    {
        Ans+=D[i]+C[i];
    }
    printf("%.2lf\n",Ans);
}

The return type of main is int not void.