How to make strings addable?

function Time(t1,t2){

t = t1 + t2
return t;
}

if t1 = 2, t2 = 4,

then i get this in return: 24, instead of 6. How do i make it add instead of append?

Code:

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function Time(t1,t2){

t = parseInt(t1) + parseInt(t2);
return t;
}

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