#include <iostream>
#include <algorithm>
#include <cstring>
using namespace std;
int dp[110][110];
int n,m,k,s;
int w[110];
int val[110];
int main ()
{
    ios::sync_with_stdio(false);
    while (cin>>n>>m>>k>>s) {
        for (int i=1;i<=k;i++)
            cin>>val[i]>>w[i];
        memset (dp,0,sizeof(dp));
        for (int i=1;i<=k;i++)
            for (int j=w[i];j<=m;j++)
                for (int t=1;t<=s;t++)
                    dp[j][t]=max (dp[j][t],dp[j-w[i]][t-1]+val[i]);
        int i;
        for (i=1;i<=m;i++)    
            if (dp[i][s]>=n) break;
        if (i>m) cout<<"-1\n";
        else      cout<<m-i<<"\n";
    }
    return 0;
}