Why do we get ambiguity errors in c++?

Hi everyone can anyone tell me
if i give 
int sum(int,int);
int sum(float,int);
int sum(float,float);
int sum(int,float);
prototypes in a pgm and give a call to sum(2,3.4);
Then an ambiguity error comes between sum(int,int) and sum(int,float);
WHY???


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