Problem in printing A*B where A and B (1≤A, B ≤10^18)

[abdalimran]

I went through a simple problem given below:

Your Task is so simple given to integer number A , B output A * B

Input:

The first line contains number of test cases T (T ≤1000)

Two Integer A and B (1≤A, B ≤10^18)

Output:

For each case, Print a case No and A * B

Sample Input/Output:
2
5 3
Case 1: 15
7 3
Case 2: 21

Problem ID: http://www.lightoj.com/practice_contest_showproblem.php?contest_id=552&problem=B

I wrote the following code for it..but it is showing Wrong Answer.

Why I'm getting WA..would anybody explain please?

#include <iostream>
#include <cstdio>

using namespace std;

int main()
{
    int t;
    long long a,b;

    scanf("%d",&t);

    for(int i=1; i<=t; i++)
    {
        scanf("%lld %lld",&a,&b);
        printf("Case %d: %lld\n",i,a*b);
    }

    return 0;
}

[cire]

// http://ideone.com/GCzHmO
#include <iostream>
#include <limits>
#include <cmath>

long long upper_limit = std::pow(10, 18);

int main()
{
    std::cout << std::numeric_limits<long long>::max() << '\n';
    std::cout << upper_limit << '\n';
    std::cout << upper_limit * upper_limit << '\n';
}

[abdalimran]

Thanks for your comment..but what did you want to make me understand? I couldn't understand. Will explain it please?

I've written a code mixing up with your given code http://ideone.com/qtCcCS but it also gives WA.

[cire]

Run the code. Read the result. Reason about what is happening.

[Smac89]

You already started this same thread somewhere else. Anyway the solution is to use a big integer class. There is one found on that website you posted
http://www.lightoj.com/article_show.php?article=1004&language=english&type=pdf