first say you have 7 stats, each with a range of 1 to 10;
then, when two or more stats share the same value, instead of adding the values one with another, you multiply them;

example with the MADEPPS system

M 4
A 3
D 3
E 5
Pr 6
Ps 5
S 3

hp= 4+3x3x3+5x5+6 = 62

now you upgrade your D which now scores a 4

hp = 4x4+3x3+5x5+6 = 56

then you upgrade again your D to a score of 5

hp = 4+3x3+5x5x5+6 = 144

...

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