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Cod sursa(job #522803)
#include<fstream.h>
ifstream f("inversmodular.in"); ofstream g("inversmodular.out");
long long a,b,q,r,ai,n,k;
long long s,s1,s2,t,t1,t2;
int main()
{f>>ai>>n; a=ai; b=n;
s1=1; t1=0;
s2=0; t2=1;
r=a%b; q=a/b;
while(r)
{s=s1-s2*q; s1=s2; s2=s;
t=t1-t2*q; t1=t2; t2=t;
a=b; b=r; r=a%b; q=a/b;
}
// ai*s+n*t=1
if(s) g<<s<<'\n';
else
{k=(-s)/n; r=(-s)%n;
if(r) k++;
g<<s+k*n<<'\n';
}
g.close(); return 0;
}