#include <bits/stdc++.h>
#define For(i, a, b) for(int (i)=(int)(a); (i)<(int)(b); ++(i))
#define rFor(i, a, b) for(int (i)=(int)(a)-1; (i)>=(int)(b); --(i))
#define rep(i, n) For((i), 0, (n))
#define rrep(i, n) rFor((i), (n), 0)
#define fi first
#define se second
using namespace std;
typedef long long lint;
typedef unsigned long long ulint;
typedef pair<int, int> pii;
typedef pair<lint, lint> pll;
template<class T> bool chmax(T &a, const T &b){if(a<b){a=b; return true;} return false;}
template<class T> bool chmin(T &a, const T &b){if(a>b){a=b; return true;} return false;}
template<class T> T div_floor(T a, T b){
    if(b < 0) a *= -1, b *= -1;
    return a>=0 ? a/b : (a+1)/b-1;
}
template<class T> T div_ceil(T a, T b){
    if(b < 0) a *= -1, b *= -1;
    return a>0 ? (a-1)/b+1 : a/b;
}

constexpr lint mod = 1000000007;
constexpr lint INF = mod * mod;
constexpr int MAX = 200010;

namespace geo{
    using real_num = double;
    constexpr real_num eps = 1e-9;
    constexpr real_num PI = 3.14159265358979323846264338327950;

    inline int sgn(real_num x){
        if(x < -eps) return -1;
        if(x > eps) return 1;
        return 0;
    }
 
    inline bool eq(real_num x, real_num y){
        return sgn(x-y) == 0;
    }
 
    inline bool ge(real_num x, real_num y){
        return sgn(x-y) == 1;
    }
 
    inline bool le(real_num x, real_num y){
        return sgn(x-y) == -1;
    }
 
    inline bool geq(real_num x, real_num y){
        return sgn(x-y) >= 0;
    }
 
    inline bool leq(real_num x, real_num y){
        return sgn(x-y) <= 0;
    }

    struct Point{
        real_num x, y;
        Point(real_num x=0, real_num y=0): x(x), y(y){}
 
        Point operator+(const Point &p){
            return {x+p.x, y+p.y};
        }
 
        Point operator-(const Point &p){
            return {x-p.x, y-p.y};
        }
 
        Point operator*(const real_num k){
            return {k*x, k*y};
        }
 
        Point operator/(const real_num k){
            return {x/k, y/k};
        }
 
        real_num operator*(const Point &p){
            return x*p.x + y*p.y;
        }
 
        real_num operator^(const Point &p){
            return x*p.y - y*p.x;
        }
 
        bool operator==(const Point &p){
            return eq(x, p.x) && eq(y, p.y);
        }
 
        bool operator<(const Point &p) const{
            if(eq(x, p.x)) return le(y, p.y);
            return le(x, p.x);
        }
    };

    using Vec = Point;
    using Points = vector<Point>;
    using Polygon = vector<Point>;

    real_num norm(Point p){
        return p.x*p.x+p.y*p.y;
    }
 
    real_num abs(Point p){
        return sqrt(norm(p));
    }
 
    real_num arg(Point p){
        return atan2(p.y, p.x);
    }

    real_num angle(Point a, Point b){
        return arg({a*b, a^b});
    }
 
    Point rot(Point p, real_num t){
        return {p.x*cos(t)-p.y*sin(t), p.x*sin(t)+p.y*cos(t)};
    }
 
    Point proj(Point a, Vec v, Point p){
        real_num t=v*(p-a)/norm(v);
        return a+v*t;
    }
 
    Point refl(Point a, Vec v, Point p){
        return proj(a, v, p)*2-p;
    }

    constexpr int CCW_COUNTER_CLOCKWISE = 1;
    constexpr int CCW_CLOCKWISE = -1;
    constexpr int CCW_ONLINE_BACK = -2; // C->A->B
    constexpr int CCW_ONLINE_FRONT = 2; // A->B->C
    constexpr int CCW_ON_SEGMENT = 0; // A->C->B
 
    inline int ccw(Point a, Point b, Point c){
        Vec v = b-a, w = c-a;
        if(ge(v^w, 0)) return CCW_COUNTER_CLOCKWISE;
        if(le(v^w, 0)) return CCW_CLOCKWISE;
        if(le(v*w, 0)) return CCW_ONLINE_BACK;
        if(le((a-b)*(c-b), 0)) return CCW_ONLINE_FRONT;
        return CCW_ON_SEGMENT;
    }

    bool is_parallel(Vec v, Vec w){
        return eq(v^w, 0);
    }

    bool is_orthogonal(Vec v, Vec w){
        return eq(v*w, 0);
    }

    bool has_intersection_ls(Point p, Vec v, Point a, Point b){
        return sgn(v^(a-p)) * sgn(v^(b-p)) <= 0;
    }

    bool has_intersection_ss(Point a, Point b, Point c, Point d){
        return ccw(a, b, c)*ccw(a, b, d) <= 0 && ccw(c, d, a)*ccw(c, d, b) <= 0;
    }

    Point intersection_ll(Point a, Vec v, Point b, Vec w){
        real_num t = ((b-a)^w) / (v^w);
        return a + v*t;
    }

    real_num distance_lp(Point a, Vec v, Point p){
        return abs(v^(p-a)/abs(v));
    }

    real_num distance_sp(Point a, Point b, Point p){
        if(le((b-a)*(p-a), 0)) return abs(p-a);
        if(le((a-b)*(p-b), 0)) return abs(p-b);
        return distance_lp(a, b-a, p);
    }

    real_num distance_ll(Point a, Vec v, Point b, Vec w){
        if(is_parallel(v, w)) return distance_lp(a, v, b);
        return 0;
    }

    real_num distance_ls(Point p, Vec v, Point a, Point b){
        if(has_intersection_ls(p, v, a, b)) return 0;
        return min(distance_lp(p, v, a), distance_lp(p, v, b));
    }

    real_num distance_ss(Point a, Point b, Point c, Point d){
        if(has_intersection_ss(a, b, c, d)) return 0;
        return min({distance_sp(a, b, c), distance_sp(a, b, d),
                distance_sp(c, d, a), distance_sp(c, d, b)});
    }

    real_num area(Polygon &p){
        real_num ret = 0;
        rep(i, p.size()) ret += p[i] ^ p[(i+1)%p.size()] / 2;
        return abs(ret);
    }

    bool is_convex(Polygon &p){
        int n = p.size();
        bool flag1 = false, flag2 = false;
        rep(i, n){
            int tmp = ccw(p[(i+n-1)%n], p[i], p[(i+1)%n]);
            if(tmp == CCW_COUNTER_CLOCKWISE){
                if(flag2) return false;
                flag1=true;
            }
            else if(tmp == CCW_CLOCKWISE){
                if(flag1) return false;
                flag2 = true;
            }
        }
        return true;
    }

    int point_in_polygon(Point a, Polygon &p){
        int n = p.size(), wn = 0;
        rep(i, n){
            int j = (i+1) % n;
            if(distance_sp(p[i], p[j], a) == 0) return 1;
            else if(p[i].y <= a.y && a.y < p[j].y){
                wn += (ccw(a, p[i], p[j]) == CCW_COUNTER_CLOCKWISE);
            }
            else if(p[j].y <= a.y && a.y < p[i].y){
                wn -= (ccw(a, p[i], p[j]) == CCW_CLOCKWISE);
            }
        }
        return wn == 0 ? 0 : 2;
    }

    Polygon convex_hull(Points p){
        int n = p.size();
        sort(p.begin(), p.end());
        Polygon ch(2*n);
        int k = 0;
        rep(i, n){
            while(k>1 && le((ch[k-1]-ch[k-2])^(p[i]-ch[k-1]), 0)) --k;
            ch[k++] = p[i];
        }
        for(int i=n-2, t=k; i>=0; --i){
            while(k>t && le((ch[k-1]-ch[k-2])^(p[i]-ch[k-1]), 0)) --k;
            ch[k++] = p[i];
        }
        ch.resize(k-1);
        return ch;
    }

    pair<real_num, pii> farthest_pair(Polygon &p){
        int n = p.size();
        if(n == 2){
            return {abs(p[0]-p[1]), {0, 1}};
        }
        int i = 0, j = 0;
        rep(k, n){
            if(le(p[k].x, p[i].x)) i = k;
            if(ge(p[k].x, p[j].x)) j = k;
        }
        real_num d = 0;
        int a = i, b = j, si = i, sj = j;
        while(i != sj || j != si){
            if(chmax(d, abs(p[i]-p[j]))) a = i, b = j;
            if(le((p[(i+1)%n]-p[i]) ^ (p[(j+1)%n]-p[j]), 0)){
                i = (i+1) % n;
            }
            else j = (j+1) % n;
        }
        return {d, {a, b}};
    }

    real_num convex_cut(Polygon &p, Point a, Vec v){
        int n = p.size();
        Polygon q;
        rep(i, n){
            int j = (i+1) % n;
            if(geq(v^(p[i]-a), 0)) q.push_back(p[i]);
            if(has_intersection_ls(a, v, p[i], p[j]) && !is_parallel(v, p[j]-p[i])){
                q.push_back(intersection_ll(a, v, p[i], p[j]-p[i]));
            }
        }
        return area(q);
    }

    pair<real_num, pii> closest_pair_rec(vector<pair<Point, int>> &p, int l, int r){
        if(r-l <= 1) return {INF, {p.size(), p.size()}};

        int m = (l+r) / 2;
        real_num x = p[m].fi.x;
        auto d = min(closest_pair_rec(p, l, m), closest_pair_rec(p, m, r));
        auto cmp = [](pair<Point, int> a, pair<Point, int> b){return a.fi.y < b.fi.y;};
        inplace_merge(p.begin()+l, p.begin()+m, p.begin()+r, cmp);

        vector<pair<Point, int>> q;
        For(i, l, r){
            if(ge(abs(p[i].fi.x-x), d.fi)) continue;
            rrep(j, q.size()){
                real_num dy = p[i].fi.y - q[j].fi.y;
                if(geq(dy, d.fi)) break;
                chmin(d, {abs(p[i].fi-q[j].fi), {p[i].se, q[j].se}});
            }
            q.push_back(p[i]);
        }
        return d;
    }

    pair<real_num, pii> closest_pair(Points &p){
        vector<pair<Point, int>> pid(p.size());
        rep(i, p.size()) pid[i] = {p[i], i};
        sort(pid.begin(), pid.end());
        return closest_pair_rec(pid, 0, p.size());
    }

    int has_intersection_cc(Point c1, real_num r1, Point c2, real_num r2){
        if(r1 < r2){
            swap(c1, c2);
            swap(r1, r2);
        }
        real_num d = abs(c1-c2), r = r1 + r2;
        if(ge(d, r)) return 4;
        if(eq(d, r)) return 3;
        if(eq(d+r2, r1)) return 1;
        if(le(d+r2, r1)) return 0;
        return 2;
    }

    bool has_intersection_cl(Point c, real_num r, Point a, Vec v){
        return leq(distance_lp(a, v, c), r);
    }

    bool has_intersection_cs(Point c, real_num r, Point a, Point b){
        return leq(distance_sp(a, b, c), r) && geq(max(abs(a-c), abs(b-c)), r);
    }

    Points intersection_cl(Point c, real_num r, Point a, Vec v){   
        Points ps;
        if(!has_intersection_cl(c, r, a, v)) return ps;
        Point p = proj(a, v, c);
        real_num t = sqrt(max((real_num)0.0, (r*r-norm(p-c)) / norm(v)));
        ps.push_back(p + v*t);
        if(!eq(t, 0)) ps.push_back(p - v*t);
        return ps;
    }

    Points intersection_cs(Point c, real_num r, Point a, Point b){
        Points ps = intersection_cl(c, r, b, a-b);
        Points qs;
        for(auto p: ps){
            if(ccw(a, b, p) == CCW_ON_SEGMENT) qs.push_back(p);
        }
        return qs;
    }

    Points intersection_cc(Point c1, real_num r1, Point c2, real_num r2){
        Points ps;
        Vec v = c2-c1, w = {v.y*-1, v.x};
        real_num d = abs(v);
        real_num x = (d*d + r1*r1 - r2*r2) / (2*d);
        real_num y = sqrt(max(r1*r1-x*x, (real_num)0.0));
        ps.push_back(c1 + v*x/d + w*y/d);
        if(has_intersection_cc(c1, r1, c2, r2)!=2) return ps;
        ps.push_back(c1 + v*x/d - w*y/d);
        return ps;
    }

    real_num common_area_ct(Point c, real_num r, Point a, Point b){
        Vec va = a-c, vb = b-c;
        if(eq(va^vb, 0)) return 0;
        else if(leq(abs(va), r) && leq(abs(vb), r)) return (va^vb)/2;
        else if(geq(distance_sp(a, b, c), r)) return r*r*angle(va, vb)/2;
        else{
            auto ps = intersection_cs(c, r, a, b);
            if(ps.size() == 1) return common_area_ct(c, r, a, ps[0]) + common_area_ct(c, r, ps[0], b);
            else return common_area_ct(c, r, a, ps[0]) + common_area_ct(c, r, ps[0], ps[1]) + common_area_ct(c, r, ps[1], b);
        }
    }

    real_num common_area_cp(Point c, real_num r, Polygon &p){
        int n = p.size();
        real_num ret = 0;
        rep(i, n){
            ret += common_area_ct(c, r, p[i], p[(i+1)%n]);
            //printf("%lf\n", common_area_ct(c, r, p[i], p[(i+1)%n]));
        }
        return ret;
    }

    real_num commn_area_cc(Point c1, real_num r1, Point c2, real_num r2){
        int flag = has_intersection_cc(c1, r1, c2, r2);
        if(flag >= 3) return 0;
        if(flag <= 1){
            real_num r = min(r1, r2);
            return PI*r*r;
        }
        real_num d = abs(c1-c2);
        real_num ret = 0;
        rep(i, 2) {
            real_num x = (d*d + r1*r1 - r2*r2) / (2*d);
            real_num t = acos(x/r1)* 2;
            ret += (t-sin(t))*r1*r1/2;
            swap(c1, c2);
            swap(r1, r2);
        }
        return ret;
    }

    Points tangent(Point c, real_num r, Point p){
        Points ps;
        real_num d = abs(p-c);
        real_num t = acos(r/d);
        ps.push_back(c + rot(p-c, t)*r/d);
        ps.push_back(c + rot(p-c, -t)*r/d);
        return ps;
    }

    Points common_tangent(Point c1, real_num r1, Point c2, real_num r2){       
        Points ps;
        int flag = has_intersection_cc(c1, r1,c2, r2);
        if(flag >= 2){
            real_num d = abs(c2-c1);
            real_num t = acos(abs(r1-r2)/d);
            if(le(r1, r2)) t = PI-t;
            ps.push_back(c1 + rot(c2-c1, t)*r1/d);
            ps.push_back(c1 + rot(c2-c1, -t)*r1/d);
        }
        if(flag == 4){
            real_num d = abs(c2-c1);
            real_num L = d*r1/(r1+r2);
            real_num t = acos(r1/L);
            ps.push_back(c1 + rot(c2-c1, t)*r1/d);
            ps.push_back(c1 + rot(c2-c1, -t)*r1/d);
        }
        if(flag == 3 || flag == 1){
            Polygon tg = intersection_cc(c1, r1, c2, r2);
            ps.push_back(tg[0]);
        }
        return ps;
    }

    Point get_o(Point a, Point b, Point c){
        Point M = (a+b)/2, N = (a+c)/2;
        Vec v = {-(b-a).y, (b-a).x}, w = {-(c-a).y, (c-a).x};
        return intersection_ll(M, v, N, w);
    }

    Point get_i(Point a, Point b, Point c){
        real_num A = abs(b-c), B = abs(c-a), C = abs(a-b);
        return (a*A+b*B+c*C)/(A+B+C);
    }

    Point get_h(Point a, Point b, Point c){
        Vec v = {-(c-b).y, (c-b).x}, w = {-(c-a).y, (c-a).x};
        return intersection_ll(a, v, b, w);
    }

    pair<Point, real_num> minimum_bounding_circle(Points &p){
        Point C;
        real_num r;
        if(p.size() == 1) C = p[0], r = 0;
        else if(p.size() == 2) C = (p[0]+p[1])/2, r = abs(p[0]-C);
        else{
            r = INF;
            Points ch = convex_hull(p);
            int K = ch.size();
            auto check = [&](Point tc, real_num tr){
                rep(i, K){
                    if(ge(abs(ch[i]-tc), tr)) return false;
                }
                return true;
            };
            rep(i, K)For(j, i+1, K){
                Point tc = (ch[i]+ch[j])/2;
                real_num tr = abs(ch[i]-tc);
                if(check(tc, tr) && chmin(r, tr)) C = tc;
                For(k, j+1, K){
                    int ccw_flag = ccw(ch[i], ch[j], ch[k]);
                    if(ccw_flag != CCW_COUNTER_CLOCKWISE && ccw_flag != CCW_CLOCKWISE) continue;
                    tc = get_o(ch[i], ch[j], ch[k]);
                    tr = abs(ch[i]-tc);
                    if(check(tc, tr) && chmin(r, tr)) C=tc;
                }
            }
        }
        return {C, r};
    }
}

using namespace geo;

int main(){
    int n;
    double r;
    scanf("%d%lf", &n, &r);
    Polygon p(n);
    rep(i, n) scanf("%lf%lf", &p[i].x, &p[i].y);
    printf("%.10lf\n", common_area_cp({0, 0}, r, p));
}


Measured memory_bytes: 3556