while solving this problem I realized that changing the direction of moving led to the new answer.

for(int i = 1; i <= n; i++)
for(int j = 1; j <= m; j++)
{...some code...}
It is WA3

for(int i = 1; i <= m; i++)
for(int j = 1; j <= n; j++)
{...some code...}
It is WA25

for(int i = n; i >= 1; i--)
for(int j = m; j >= 1; j--)
{...some code...}
It is also WA25

for(int i = m; i >= 1; i--)
for(int j = n; j >= 1; j--)
{...some code...}
It is AC.

So, if you have wa25 or wa3 you may try to change indexe's direction )

This means weak tests. But I wonder if there is a test which does not give result with all 4 directions.