Language: C++

#include <bits/stdc++.h>
using namespace std;

#define rep(i,x,y) for(int i=(x);i<(y);++i)
#define debug(x) #x << "=" << (x)

#ifdef DEBUG
#define _GLIBCXX_DEBUG
#define show(x) std::cerr << debug(x) << " (L:" << __LINE__ << ")" << std::endl
#else
#define show(x)
#endif

typedef long long int ll;
typedef pair<int,int> pii;
template<typename T> using vec=std::vector<T>;

const int inf=1<<30;
const long long int infll=1LL<<62;
const double eps=1e-9;
const int dx[]={1,0,-1,0},dy[]={0,1,0,-1};

template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec){
    os << "[";
    for (const auto &v : vec) {
    	os << v << ",";
    }
    os << "]";
    return os;
}

typedef complex<long double> point;
typedef pair<point,point> line;
bool equal(const long double x1,const long double x2){ return abs(x1-x2)<eps; }
bool equal(const point& a,const point& b){ return equal(a.real(),b.real()) and equal(a.imag(),b.imag()); }
long double length(const point& a){ return abs(a); }
long double distance(const point& a,const point& b){ return abs(a-b); }
long double dot(const point& a,const point& b){ return a.real()*b.real()+a.imag()*b.imag(); }
long double cross(const point& a,const point& b){ return a.real()*b.imag()-a.imag()*b.real(); }
bool is_orthogonal(const point& a1,const point& a2,const point& b1,const point& b2){ return equal(dot(a1-a2,b1-b2),0.0); }
bool is_orthogonal(const line& l,const line& m){ return is_orthogonal(l.first,l.second,m.first,m.second); }
bool is_parallel(const point& a1,const point& a2,const point& b1,const point& b2){ return equal(cross(a1-a2, b1-b2),0.0); }
bool is_parallel(const line& l,const line& m){ return is_parallel(l.first,l.second,m.first,m.second); }
int ccw(point a,point b,point c) {
    b-=a; c-=a;
    if(cross(b, c) > 0)   return +1;       // counter clockwise
    if(cross(b, c) < 0)   return -1;       // clockwise
    if(dot(b, c) < 0)     return +2;       // c--a--b on line
    if(norm(b) < norm(c)) return -2;       // a--b--c on line
    return 0;
}
bool intersectLL(const line& l, const line& m){
    return abs(cross(l.second-l.first, m.second-m.first)) > eps || // non-parallel
           abs(cross(l.second-l.first, m.first-l.first)) < eps;   // same line
}
bool intersectLS(const line& l, const line& s){
    return cross(l.second-l.first, s.first-l.first)*       // s.first is left of l
           cross(l.second-l.first, s.second-l.first) < eps; // s.second is right of l
}
bool intersectLP(const line& l, const point& p){
    return abs(cross(l.second-p, l.first-p)) < eps;
}
bool intersectSS(const line& s, const line& t){
    return ccw(s.first,s.second,t.first)*ccw(s.first,s.second,t.second) <= 0 &&
           ccw(t.first,t.second,s.first)*ccw(t.first,t.second,s.second) <= 0;
}
bool intersectSP(const line& s, const point& p){
    return abs(s.first-p)+abs(s.second-p)-abs(s.second-s.first) < eps; // triangle inequality
}
point projection(const line& l, const point& p) {
    long double t = dot(p-l.first, l.first-l.second) / norm(l.first-l.second);
    return l.first + t*(l.first-l.second);
}
point reflection(const line& l, const point& p){
    return p+point((projection(l, p) - p).real()*2,(projection(l, p) - p).imag()*2);
}
long double distanceLP(const line &l, const point& p){
    return abs(p - projection(l, p));
}
long double distanceLL(const line &l, const line &m){
    return intersectLL(l, m) ? 0 : distanceLP(l, m.first);
}
long double distanceLS(const line &l, const line &s){
    if (intersectLS(l, s)) return 0;
    return min(distanceLP(l, s.first), distanceLP(l, s.second));
}
long double distanceSP(const line &s, const point& p) {
    const point r = projection(s, p);
    if (intersectSP(s, r)) return abs(r - p);
    return min(abs(s.first - p), abs(s.second - p));
}
long double distanceSS(const line &s, const line &t) {
    if (intersectSS(s, t)) return 0;
    return min(min(distanceSP(s, t.first), distanceSP(s, t.second)),
             min(distanceSP(t, s.first), distanceSP(t, s.second)));
}

point intersection_l(const point& a1,const point& a2,const point& b1,const point& b2) {
  const point& a=a2-a1,b=b2-b1;
  return a1+a*cross(b, b1-a1)/cross(b, a);
}
point intersection_l(const line& l,const line& m){
    return intersection_l(l.first,l.second,m.first,m.second);
}

void solve(int w,int h,int n){
    vector<point> ps;
    ps.push_back(point(0,0));
    ps.push_back(point(0,h));
    ps.push_back(point(w,0));
    ps.push_back(point(w,h));
    rep(i,0,2*n){
        long double x,y;
        cin >> x >> y;
        if(x==0) x=1e-10;
        ps.push_back(point(x,y));
    }

    vector<long double> ys;
    line l1(point(0,0),point(0,h));
    rep(i,0,ps.size()) rep(j,i+1,ps.size()){
        line l(ps[i],ps[j]);
        if(is_parallel(l1,l)) continue;
        point p=intersection_l(l1,l);
        if(p.imag()<0 or h<p.imag()) continue;
        ys.push_back(p.imag());
    }
    sort(ys.begin(),ys.end());
    ys.erase(unique(ys.begin(),ys.end()),ys.end());

    long double sum=0;
    line l2(point(w,0),point(w,h));
    rep(i,0,ys.size()-1){
        point p1(0,ys[i]+(ys[i+1]-ys[i])/2);
        vector<long double> ys2;
        rep(j,4,ps.size()){
            auto &p2=ps[j];
            line l(p1,p2);
            point p=intersection_l(l,l2);
            ys2.push_back(p.imag());
        }
        sort(ys2.begin(),ys2.end());
        sum+=(min((long double)h,ys2[n])-max((long double)0,ys2[n-1]))*(ys[i+1]-ys[i]);s
    }
    cout << sum/h/h << endl;
}

int main(){
    std::cin.tie(0);
    std::ios::sync_with_stdio(false);
    cout.setf(ios::fixed);
    cout.precision(12);
    while(true){
        int w,h,n;
        cin >> w >> h >> n;
        if(!w and !h and !n) break;
        solve(w,h,n);
    }
    return 0;
}

Compiler output:
a.cc: In function 'void solve(int, int, int)':
a.cc:143:88: error: 's' was not declared in this scope
  143 |         sum+=(min((long double)h,ys2[n])-max((long double)0,ys2[n-1]))*(ys[i+1]-ys[i]);s
      |                                                                                        ^