Ниточка

#define _USE_MATH_DEFINES
#include <iostream>
#include <conio.h>
#include <string>
#include <set>
#include <map>
#include <algorithm>
#include <string>
#include <math.h>
#include <vector>
#include <cmath>

using namespace std;

struct cord
{
    double x, y;
};

int main(void)
{
    cord data[101];
    double line = 0;
    double r, n;

    cin >> n >> r;

    cout.setf(ios::fixed);
    cout.precision(2);
    line = r*M_PI*2;

    for (int i = 1; i <= n; i++)
        cin >> data[i].x >> data[i].y;

    for (int i = 1; i <= n; i++)
        if (i != n)
            line = line + sqrt(abs(pow(data[i].x - data[i + 1].x, 2) + pow(data[i].y - data[i + 1].y, 2)));
        else
            line = line + sqrt(abs(pow(data[i].x - data[1].x, 2) + pow(data[i].y - data[1].y, 2)));

    cout << line;

    _getch();
    return 0;
}