function Main(input) {
	input = input.split(" ");
	var tHP = parseInt(input[0], 10);
	var tAT = parseInt(input[1], 10);
	var aHP = parseInt(input[2], 10);
	var aAT = parseInt(input[3], 10);

while(true){
aHP =  aHP  -  tAT 
if ( aHP <= 0 ){console.log('yes'); break;}
tHP =  tHP  -  aAT 
if ( tHP <= 0 ){console.log('No');  break;}

}
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));