#include<stdio.h>
#define maxn 1003
long n,i;
long long v[maxn],x,l,a,d,s,t;
FILE *f, *g;
int main()
{f=fopen("lupu.in","r");
g=fopen("lupu.out","w");
fscanf(f,"%ld %lld %lld",&n,&x,&l);
for(i=1;i<=n;i++)
{fscanf(f,"%lld %lld",&d,&a);
if(d<=a) {t=(x-d)/l+1;
while(t)
{if(v[t]<a)
{v[t]=a;
t=0;}
else t--;}}}
for(i=1;i<=n;i++)
s+=v[i];
fprintf(g,"%lld",s);
fclose(f);
fclose(g);
return 0;}