1353A - Most Unstable Array.cpp

/*
You are given two integers n and m. You have to construct the array a of length n consisting of non-negative integers (i.e. integers greater than or equal to zero) such that the sum of elements of this array is exactly m and the value ?i=1n-1|ai-ai+1| is the maximum possible. Recall that |x| is the absolute value of x.

In other words, you have to maximize the sum of absolute differences between adjacent (consecutive) elements. For example, if the array a=[1,3,2,5,5,0] then the value above for this array is |1-3|+|3-2|+|2-5|+|5-5|+|5-0|=2+1+3+0+5=11. Note that this example doesn't show the optimal answer but it shows how the required value for some array is calculated.

You have to answer t independent test cases.

Input
The first line of the input contains one integer t (1=t=104) — the number of test cases. Then t test cases follow.

The only line of the test case contains two integers n and m (1=n,m=109) — the length of the array and its sum correspondingly.

Output
For each test case, print the answer — the maximum possible value of ?i=1n-1|ai-ai+1| for the array a consisting of n non-negative integers with the sum m.

Example
inputCopy
5
1 100
2 2
5 5
2 1000000000
1000000000 1000000000
outputCopy
0
2
10
1000000000
2000000000
Note
In the first test case of the example, the only possible array is [100] and the answer is obviously 0.

In the second test case of the example, one of the possible arrays is [2,0] and the answer is |2-0|=2.

In the third test case of the example, one of the possible arrays is [0,2,0,3,0] and the answer is |0-2|+|2-0|+|0-3|+|3-0|=10.
*/



#include<bits/stdc++.h>
using namespace std;

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    
    int t;
    cin >> t;
    
    while (t--) {
    	int n, m;
    	cin >> n >> m;
    	
    	if (n == 1) cout << 0 << endl;
    	else if (n == 2) cout << m << endl;
    	else cout << 2 * m << endl;
	}
  	
    return 0;
}