#include <cstdio>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/hash_policy.hpp>
using namespace std;
int read(){
	char c=getchar();int x=0;
	while(c<48||c>57) c=getchar();
	do x=(x<<1)+(x<<3)+(c^48),c=getchar();
	while(c>=48&&c<=57);
	return x;
}
typedef long long ll;
const ll INF=0x3f3f3f3f3f3f3f3f;
const int N=503;
__gnu_pbds::gp_hash_table<int,ll> mp;
int s,d,t,n,dt;
void chmn(ll &x,ll v){if(x>v) x=v;}
ll calc(int x){
	if(x<0) return 0;
	ll res=x;
	chmn(res,(ll)(x+d-1)/d*t);
	chmn(res,(ll)(x/d)*t+x%d);
	return res;
}
int l[N],r[N];
ll dfs(int x){
	if(x>=s) return 0;
	if(mp.find(x)!=mp.end()) return mp[x];
	int pos=lower_bound(l+1,l+n+1,x)-l;
	if(pos>n) return mp[x]=calc(s-x);
	ll res=INF;
	if(x+d<=l[pos]){
		int t=(l[pos]-x)/d;
		chmn(res,dfs(x+t*d)+(ll)t*dt);
	}
	else{
		int ver=x+d;
		int o=upper_bound(r+1,r+n+1,ver)-r;
		if(o<=n&&l[o]<ver) ver=l[o];
		chmn(res,dfs(ver)+t);
	}
	for(int i=pos;i<=n;++i){
		if(l[i]-d>l[pos]) break;
		if(l[i]>x) chmn(res,dfs(l[i])+calc(l[i]-d-x)+t);
		if(r[i]-d>l[pos]) break;
		if(r[i]>x) chmn(res,dfs(r[i])+calc(r[i]-d-x)+t);
	}
	return mp[x]=res;
}
int main(){
	//freopen("kun.in","r",stdin);
	//freopen("kun.out","w",stdout);
	s=read();d=read();t=read();n=read();dt=t;if(d<dt) dt=d;
	for(int i=1;i<=n;++i){l[i]=read();r[i]=read();}
	printf("%lld\n",dfs(0));
	return 0;
}