#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define rep(i,a,b) for(int i=a;i<=b;i++)
map<ll, ll> f;
ll l[5005], r[5005], s, d, t, n;
ll dfs(ll st) {
	if (st >= s) return 0;
	ll ans = 1e18;
	if (f.count(st)) return f[st];
	ll mx = st + d, id = 0;
	rep(i, 1, n + 1) {
		if (l[i] < mx && mx < r[i]) mx = l[i];
		if (l[i] >= st && !id) id = i;
	}
	ll num = (r[id] - d - st) / d;
	if (num > 0) {
		f[st] = dfs(st + num * d) + num * min(d, t);
		return f[st];
	}
	rep(i, id, n + 1)
	if (l[id] + d >= r[i]) ans = min(ans, min(t, max(0ll, r[i] - d - st)) + t + dfs(r[i]));
	else break;
	if (id == n + 1) ans = min(ans, s - st);
	f[st] = ans;
	return ans;
}
signed main() {
	cin >> s >> d >> t >> n;
	rep(i, 1, n) cin >> l[i] >> r[i];
	l[n + 1] = r[n + 1] = s;
	cout << dfs(0);
	return 0;
}