B.cpp

#include <bits/stdc++.h>

#define mp make_pair
#define F first
#define S second

using namespace std;

typedef long long ll;
typedef long double ld;

using pll = pair<ll, ll>;

pll operator+(pll a, pll b){
    return mp(a.F + b.F, a.S + b.S);
}

pll operator-(pll a, pll b){
    return mp(a.F - b.F, a.S - b.S);
}

ll cross(pll a, pll b){
    return a.F * b.S - a.S * b.F;
}

ld dis(pll a, pll b){
    ll x = a.F - b.F, y = a.S - b.S;
    return sqrt(x * x + y * y);
}

ld absv(pll v){
    return sqrt(v.F * v.F + v.S * v.S);
}

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(0);

    ll x, y, n;
    cin >> x >> y >> n;
    pll a = mp(x, y);

    pll c;
    cin >> c.F >> c.S;

    ld ans = dis(a, c);
    for(int i = 1; i <= n; i++){
        pll d;
        cin >> d.F >> d.S;

        pll cd = d - c;
        pll v = mp(cd.S, -cd.F);
        pll ca = a - c;

        ld len = (ld)cross(ca, cd) / cross(cd, v);
        ld px = a.F + len * v.F;
        ld py = a.S + len * v.S;

        ll mnx = min(c.F, d.F), mxx = max(c.F, d.F);
        ll mny = min(c.S, d.S), mxy = max(c.S, d.S);

        if(mnx <= px && px <= mxx && mny <= py && py <= mxy)
            ans = min(ans, abs(len) * absv(v));

        ans = min(ans, dis(a, d));
        c = d;
    }

    cout << fixed << setprecision(20) << ans << "\n";

    return 0;
}