function erf(x) {
        var a1 = 0.254829592;
        var a2 = -0.284496736;
        var a3 = 1.421413741;
        var a4 = -1.453152027;
        var a5 = 1.061405429;
        var p = 0.3275911;
        var sign = 1;
        if (x < 0) {
            sign = -1;
        }
        x = Math.abs(x);
        var t = 1.0 / (1.0 + p * x);
        var y = 1.0 - (((((a5 * t + a4) * t) + a3) * t + a2) * t + a1) * t * Math.exp(-x * x);
        return sign * y;
    }
     let arr = input.split(/[\n]/g);
     let st = (parseFloat(arr[0]) - (parseInt(arr[1]) * parseFloat(arr[2]))) / (Math.sqrt(parseInt(arr[1])) * parseFloat(arr[3]));
     let ans = (1.0 + erf(st / Math.sqrt(2.0))) / 2.0;
    console.log(ans.toFixed(4));