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int modular(int n, int low, int high) {
    return low + (n + high - low + 1) % (high - low + 1);
}

unittest {
    assert(modular(0,100,200)   == 100);
    assert(modular(50,100,200)  == 150);
    assert(modular(100,100,200) == 200);
    assert(modular(101,100,200) == 100);
    assert(modular(-1,100,200)  == 200);
    assert(modular(1,-5,200)    ==  -4);
    assert(modular(-500,-5,-1)  ==  -5);
}