I got WA 10, where is wrong?


///////////////////////
#include <iostream>
using namespace std;
int main()
{
    int N,M,sum,counter,mul;
    counter=0;
    cin>>N>>M;
    sum=M;
    mul=N*M;
    while(sum!=mul)
    {
        N--;
        sum+=N;
        counter++;
        if(sum==mul)
            break;
        M--;
        sum+=M;
        counter++;
    }
    cout<<counter<<endl;

}