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#include <iostream>
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using namespace std;
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int main() {
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    double V,T,S,D,hittime;
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    cin>>V>>T>>S>>D;
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    hittime=D/V;
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    if(hittime<T || hittime>S)
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    {
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        cout<<"Yes";
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    }
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    else
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    {
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        cout<<"No";
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    }
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    return 0;
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}