#include<bits/stdc++.h>
using namespace std;

typedef long long int       ll;
typedef unsigned long long int  ull;
typedef vector<int>         vi;
typedef vector<ll>          vll;
typedef vector<vi>          vvi;
typedef pair<int,int >      pii;
typedef vector< pii >       vpii;
typedef set<int>            sti;

#define sc                  scanf
#define pf                  printf
#define sci(n)              scanf("%d",&n)
#define scl(n)              scanf("%lld",&n)
#define scf(n)              scanf("%lf",&n)
#define scs(s)              scanf("%s",s)
#define pfi(n)              printf("%d",n)
#define pfl(n)              printf("%lld",n)
#define pff(n)              cout<<n
#define line                printf("\n")
#define spc                 printf(" ")
#define loop(i,x,y)         for(int i=int(x); i<=int(y); i++)
#define rloop(i,y,x)        for(int i=int(y); i>=int(x); i--)
#define cspf(i)             printf("Case %d: ", i)
#define pb                  push_back
#define mp                  make_pair
#define ff                  first
#define ss                  second
#define all(v)              v.begin(),v.end()
#define rall(v)             v.rbegin(),v.rend()
#define read()              freopen("input.txt", "r", stdin)
#define write()             freopen("output.txt", "w", stdout)
/// Constants
#define eps                 1e-9
#define PI                  acos(-1.0)  // 3.1415926535897932

int main()
{
    int n,l,a,sm=0;
    int t[100009],k[100009],y[100009];
    sci(n);sci(l),sci(a);
    loop(i,0,n-1){
        sci(t[i]);sci(k[i]);
        y[i]=t[i]+k[i];
        if(!i){
            int diff=t[i]/a;
            sm+=diff;
        }
        else{
            int diff=(t[i]-y[i-1]);
            sm+=(diff/a);
        }
    }
    int dif=l-y[n-1];
    sm+=(dif/a);
    pfi(sm);line;
    return 0;
}