Is there a way I can run a function or update score just once without using update function?

For Example.

int score = 0;
private bool missionSuccess=false;

private void Update()
{
Setvalue(5);
}
private void Setvalue(int value)
{
if(missionSuccess)
score += value;
}[/code]


if missionSuccess= true then the score will dramatically the value of the score every frame. What if I just want to add once when the bool missionSuccess turns true? can it be possibly solved by using delegates or event systems?