import java.math.BigInteger; public class Euler { public static void main(String args[]) { BigInteger a = BigInteger.valueOf(1); BigInteger b = BigInteger.valueOf(3); while (true) { BigInteger c = b.multiply(BigInteger.valueOf(6)).subtract(a).subtract(BigInteger.valueOf(2)); BigInteger t = c.multiply(c).subtract(c).multiply(BigInteger.valueOf(8)).add(BigInteger.ONE); t = BigInteger.valueOf((long) ((Math.sqrt(t.doubleValue()) + 1.000000005) / 2)); if (t.longValue() > 1000000000000L) { System.out.println(c); return; } a = b; b = c; } } }