function calcCost(data) {
    const [
        tariffCost,
        trafficLimit,
        extraMegabyteCost,
        spentTraffic
    ] = data.toString().trim().split(/\s+/).map(Number);

    if (trafficLimit >= spentTraffic) return tariffCost;

    return tariffCost + extraMegabyteCost * (spentTraffic - trafficLimit);
}


var readline = require('readline');
var rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
});

let result = 0;

process.stdin.on('end', () => {
    console.log(result);
    process.exit(0);
});

rl.on('line', function (data) {
    result = calcCost(data);
});