It's 100, Roughly Speaking

Logic Level 1

$\large9 \ \boxed{\phantom0} \ 15 \ \boxed{\phantom0} \ 57\ \boxed{\phantom0} \ 77\ \boxed{\phantom0} \ 96 = 100$

Each box in the expression above can be replaced by the mathematical operators $$+$$ or $$-$$. How many times was the operator $$+$$ used to makes the expression equal to 100?

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