C++ :

#include <cstdio>
int main(){
    long long X,Y,M;
    if(scanf("%lld %lld %lld",&X,&Y,&M)!=3) return 0;
    long long best=0;
    for(long long a=0; a*X<=M; a++){
        long long r = M-a*X;
        long long v = a*X + (r/Y)*Y;
        if(v>best) best=v;
    }
    printf("%lld\n", best);
    return 0;
}