#include<bits/stdc++.h>
using namespace std;
inline long long read(){
	long long ret=0,neg=1;
	char c;
	c=getchar();
	while((c<'0'||c>'9')&&c!='-'){
		c=getchar();
	}
	if(c=='-'){
		neg=-1;
		c=getchar();
	}
	while(c>='0'&&c<='9'){
		ret=(ret<<1)+(ret<<3)+c-'0';
		c=getchar();
	}
	return ret*neg;
}
void write(long long x){
	if(x<0){
		x=-x;
		putchar('-');
	}
	if(x>=10) write(x/10);
	putchar((x%10)+'0');
}
long long s,d,t,n;
long long lb[510],rb[510];
long long r[1100],ridx;
long long f[2000010],pos[2000010],idx;
inline void Addr(long long del){
	++ridx;
	r[ridx]=del;
}
inline bool vldp(long long pp){
	//cout<<pp<<endl;
	if(pp<0||pp>s) return false;
	/*int ll,rr,mid;
	ll=1,rr=n+1;
	while(ll<rr){
		mid=(ll+rr)>>1;
		if(rb[mid]<=pp) ll=mid+1;
		else rr=mid;
	}*/
	int ll=upper_bound(rb+1,rb+n+2,pp)-rb;
	if(ll==n+1) return true;
	if(pp<=lb[ll]) return true;
	return false;
}
inline void Addp(long long pp){
	for(int i=1;i<=ridx;++i){
		long long rr=(pp/d);
		rr=rr*d;
		//left
		long long tmp;
		tmp=rr+r[i];
		if(tmp>pp) tmp-=d;
		if(vldp(tmp)){
			++idx;
			pos[idx]=tmp;
		}
		//right
		tmp=rr+r[i];
		if(tmp<pp) tmp+=d;
		if(vldp(tmp)){
			++idx;
			pos[idx]=tmp;
		}
	}
}
map<long long,long long> mp;
int lstf[1100];
int main(){
	//freopen("kun2.in","r",stdin);
	//freopen("kun.out","w",stdout);
	s=read();
	d=read();
	t=read();
	n=read();
	Addr(0);
	Addr(s%d);
	for(int i=1;i<=n;++i){
		lb[i]=read();
		rb[i]=read();
		Addr(lb[i]%d);
		Addr(rb[i]%d);
	}
	/*cout<<ridx<<endl;
	for(int i=1;i<=ridx;++i){
		cout<<r[i]<<' ';
	}
	cout<<endl;*/
	sort(r+1,r+1+ridx);
	ridx=unique(r+1,r+1+ridx)-r-1;
	/*cout<<ridx<<endl;
	for(int i=1;i<=ridx;++i){
		cout<<r[i]<<' ';
	}*/
	for(int i=1;i<=ridx;++i){
		mp[r[i]]=i;
	}
	rb[n+1]=s+10;
	Addp(0);
	Addp(s);
	for(int i=1;i<=n;++i){
		Addp(lb[i]);
		Addp(rb[i]);
	}
	sort(pos+1,pos+1+idx);
	idx=unique(pos+1,pos+1+idx)-pos-1;
	/*for(int i=1;i<=idx;++i){
		cout<<pos[i]<<' ';
	}*/
	//memset(f,0x3f,sizeof(f));
	for(int i=0;i<=idx;++i){
		f[i]=10000000000000000;
	}
	f[1]=0;
	multiset<long long> ms;
	int dlst,lsti;
	lsti=0;
	dlst=0;
	//assert(0);
	for(int i=1;i<=idx;++i){
		//cout<<pos[i]<<' ';
		while(lsti+1<=n&&rb[lsti+1]<=pos[i]) ++lsti;
		while(dlst<=idx&&pos[i]-pos[dlst]>d){
			if(dlst>0){
				ms.erase(ms.find(f[dlst]));
			}
			++dlst;
		}
		if(rb[lsti]<=pos[i-1]) f[i]=min(f[i],f[i-1]+pos[i]-pos[i-1]);
		if(ms.begin()!=ms.end()){
			f[i]=min(f[i],(*ms.begin())+t);
		}
		int cr=pos[i]%d;
		cr=mp[cr];
		if(lstf[cr]&&rb[lsti]<=pos[lstf[cr]]){
			f[i]=min(f[i],f[lstf[cr]]+((pos[i]-pos[lstf[cr]])/d)*t);
		}
		ms.insert(f[i]);
		lstf[cr]=i;
		//cout<<pos[i]<<' '<<f[i]<<endl;
	}
	write(f[idx]);
	return 0;
}